doc: add information about modules cache behavior

This publicly documents that adding native module names will resolve
the added entry instead of the native module.

It also updates the description why extensions are deprecated.

PR-URL: #26971
Refs: #25362
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>

doc/api/modules.md

@@ -196,28 +196,26 @@ NODE_MODULES_PATHS(START)
 
 <!--type=misc-->
 
-Modules are cached after the first time they are loaded. This means
-(among other things) that every call to `require('foo')` will get
-exactly the same object returned, if it would resolve to the same file.
+Modules are cached after the first time they are loaded. This means (among other
+things) that every call to `require('foo')` will get exactly the same object
+returned, if it would resolve to the same file.
 
-Provided `require.cache` is not modified, multiple calls to
-`require('foo')` will not cause the module code to be executed multiple times.
-This is an important feature. With it, "partially done" objects can be returned,
-thus allowing transitive dependencies to be loaded even when they would cause
-cycles.
+Provided `require.cache` is not modified, multiple calls to `require('foo')`
+will not cause the module code to be executed multiple times. This is an
+important feature. With it, "partially done" objects can be returned, thus
+allowing transitive dependencies to be loaded even when they would cause cycles.
 
-To have a module execute code multiple times, export a function, and call
-that function.
+To have a module execute code multiple times, export a function, and call that
+function.
 
 ### Module Caching Caveats
 
 <!--type=misc-->
 
-Modules are cached based on their resolved filename. Since modules may
-resolve to a different filename based on the location of the calling
-module (loading from `node_modules` folders), it is not a *guarantee*
-that `require('foo')` will always return the exact same object, if it
-would resolve to different files.
+Modules are cached based on their resolved filename. Since modules may resolve
+to a different filename based on the location of the calling module (loading
+from `node_modules` folders), it is not a *guarantee* that `require('foo')` will
+always return the exact same object, if it would resolve to different files.
 
 Additionally, on case-insensitive file systems or operating systems, different
 resolved filenames can point to the same file, but the cache will still treat
@@ -412,7 +410,7 @@ are not found elsewhere.
 On Windows, `NODE_PATH` is delimited by semicolons (`;`) instead of colons.
 
 `NODE_PATH` was originally created to support loading modules from
-varying paths before the current [module resolution][] algorithm was frozen.
+varying paths before the current [module resolution][] algorithm was defined.
 
 `NODE_PATH` is still supported, but is less necessary now that the Node.js
 ecosystem has settled on a convention for locating dependent modules.
@@ -585,6 +583,11 @@ value from this object, the next `require` will reload the module. Note that
 this does not apply to [native addons][], for which reloading will result in an
 error.
 
+Adding or replacing entries is also possible. This cache is checked before
+native modules and if a name matching a native module is added to the cache,
+no require call is
+going to receive the native module anymore. Use with care!
+
 #### require.extensions
 <!-- YAML
 added: v0.3.0
@@ -603,22 +606,13 @@ Process files with the extension `.sjs` as `.js`:
 require.extensions['.sjs'] = require.extensions['.js'];
 ```
 
-**Deprecated.** In the past, this list has been used to load
-non-JavaScript modules into Node.js by compiling them on-demand.
-However, in practice, there are much better ways to do this, such as
-loading modules via some other Node.js program, or compiling them to
-JavaScript ahead of time.
-
-Since the module system is locked, this feature will probably never go
-away. However, it may have subtle bugs and complexities that are best
-left untouched.
-
-Note that the number of file system operations that the module system
-has to perform in order to resolve a `require(...)` statement to a
-filename scales linearly with the number of registered extensions.
+**Deprecated.** In the past, this list has been used to load non-JavaScript
+modules into Node.js by compiling them on-demand. However, in practice, there
+are much better ways to do this, such as loading modules via some other Node.js
+program, or compiling them to JavaScript ahead of time.
 
-In other words, adding extensions slows down the module loader and
-should be discouraged.
+Avoid using `require.extensions`. Use could cause subtle bugs and resolving the
+extensions gets slower with each registered extension.
 
 #### require.main
 <!-- YAML