[skip changelog] Correct documentation re: default for build_cache.path configuration key (#2919)

The "Configuration" documentation page includes information on the default values of the configuration keys.

The default value of the build_cache.path configuration key was recently changed, but the documentation was not updated
accordingly.

The documentation is hereby updated to provide the correct information regarding the default value of the
build_cache.path configuration key.

Previously, the information was provided in the list of keys. That is a non-standard location for the information as
there is an established convention of instead providing this information in the separate section of the page dedicated
to documenting default paths. In addition. the location in the keys list became more problematic for the new default
since it is operating system-specific and thus the increased volume of documentation content would be awkward to fit
into the nested list of keys. For these reasons, the documentation content was moved to the standardized location under
the "Default directories" section of the page.

Author: per1234

docs/configuration.md

@@ -38,7 +38,7 @@
 - `updater` - configuration options related to Arduino CLI updates
   - `enable_notification` - set to `false` to disable notifications of new Arduino CLI releases, defaults to `true`
 - `build_cache` configuration options related to the compilation cache
-  - `path` - the path to the build cache, default is `$TMP/arduino`.
+  - `path` - the path to the folder under which build caches are stored.
   - `extra_paths` - a list of paths to look for precompiled artifacts if not found on `build_cache.path` setting.
   - `compilations_before_purge` - interval, in number of compilations, at which the cache is purged, defaults to `10`.
     When `0` the cache is never purged.
@@ -58,6 +58,14 @@
 The following are the default directories selected by the Arduino CLI if alternatives are not specified in the
 configuration file.
 
+- The `build_cache.path` default is OS-dependent:
+
+  - on Linux (and other Unix-based OS) is: if
+    [`$XDG_CACHE_HOME`](https://specifications.freedesktop.org/basedir-spec/latest/#variables) is defined,
+    `$XDG_CACHE_HOME/arduino`. Otherwise `{HOME}/.config/arduino`.
+  - on Windows is: `{HOME}/AppData/Local/arduino`
+  - on MacOS is: `{HOME}/Library/Caches/arduino`
+
 - The `directories.data` default is OS-dependent:
 
   - on Linux (and other Unix-based OS) is: `{HOME}/.arduino15`