cmd/go: add documentation for the -overlay flag

Also add -overlay to the Go 1.16 release notes.

For #40700
Fixes #39958
Fixes #42893

Change-Id: Ifd397549e368b255e7b8800986cfa0563a942af5
Reviewed-on: https://go-review.googlesource.com/c/go/+/274714
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

doc/go1.16.html

@@ -271,6 +271,20 @@ <h4 id="list-buildid">The <code>list</code> command</h4>
   but without the extra step.
 </p>
 
+<h4 id="overlay-flag">The <code>-overlay</code> flag</h4>
+
+<p><!-- golang.org/issue/39958 -->
+  The <code>-overlay</code> flag specifies a JSON configuration file containing
+  a set of file path replacements. The <code>-overlay</code> flag may be used
+  with all build commands and <code>go</code> <code>mod</code> subcommands.
+  It is primarily intended to be used by editor tooling such as gopls to
+  understand the effects of unsaved changes to source files.  The config file
+  maps actual file paths to replacement file paths and the <code>go</code>
+  command and its builds will run as if the actual file paths exist with the
+  contents given by the replacement file paths, or don't exist if the replacement
+  file paths are empty.
+</p>
+
 <h3 id="cgo">Cgo</h3>
 
 <p><!-- CL 252378 -->

src/cmd/go/internal/work/build.go

@@ -124,6 +124,17 @@ and test commands:
 		directory, but it is not accessed. When -modfile is specified, an
 		alternate go.sum file is also used: its path is derived from the
 		-modfile flag by trimming the ".mod" extension and appending ".sum".
+	-overlay file
+		read a JSON config file that provides an overlay for build operations.
+		The file is a JSON struct with a single field, named 'Replace', that
+		maps each disk file path (a string) to its backing file path, so that
+		a build will run as if the disk file path exists with the contents
+		given by the backing file paths, or as if the disk file path does not
+		exist if its backing file path is empty. Support for the -overlay flag
+		has some limitations:importantly, cgo files included from outside the
+		include path must be  in the same directory as the Go package they are
+		included from, and overlays will not appear when binaries and tests are
+		run through go run and go test respectively.
 	-pkgdir dir
 		install and load all packages from dir instead of the usual locations.
 		For example, when building with a non-standard configuration,