doc: fix/add link to Android info

We have two notes in API docs about Android support:
the first has no links, the second links to the table of supported OSs
where Android is not mentioned which may be confusing.

This PR makes both notes link to dedicated Android part of BUILDING.md.

PR-URL: nodejs/node#19004
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

doc/api/os.md

@@ -324,7 +324,7 @@ Equivalent to [`process.platform`][].
 
 *Note*: The value `'android'` may also be returned if the Node.js is built on
 the Android operating system. However, Android support in Node.js is considered
-to be experimental at this time.
+[to be experimental][Android building] at this time.
 
 ## os.release()
 <!-- YAML
@@ -1185,4 +1185,5 @@ The following error codes are specific to the Windows operating system:
 
 [`process.arch`]: process.html#process_process_arch
 [`process.platform`]: process.html#process_process_platform
+[Android building]: https://github.com/nodejs/node/blob/master/BUILDING.md#androidandroid-based-devices-eg-firefox-os
 [uname(3)]: https://linux.die.net/man/3/uname

doc/api/process.md

@@ -1394,7 +1394,7 @@ console.log(`This platform is ${process.platform}`);
 
 The value `'android'` may also be returned if the Node.js is built on the
 Android operating system. However, Android support in Node.js
-[is experimental][Supported platforms].
+[is experimental][Android building].
 
 ## process.ppid
 <!-- YAML
@@ -1921,6 +1921,7 @@ cases:
 [`promise.catch()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch
 [`require.main`]: modules.html#modules_accessing_the_main_module
 [`setTimeout(fn, 0)`]: timers.html#timers_settimeout_callback_delay_args
+[Android building]: https://github.com/nodejs/node/blob/master/BUILDING.md#androidandroid-based-devices-eg-firefox-os
 [Child Process]: child_process.html
 [Cluster]: cluster.html
 [debugger]: debugger.html
@@ -1932,6 +1933,5 @@ cases:
 [Readable]: stream.html#stream_readable_streams
 [Signal Events]: #process_signal_events
 [Stream compatibility]: stream.html#stream_compatibility_with_older_node_js_versions
-[Supported platforms]: https://github.com/nodejs/node/blob/master/BUILDING.md#supported-platforms-1
 [TTY]: tty.html#tty_tty
 [Writable]: stream.html#stream_writable_streams