Bug 1863497 - sccache: update the doc r=glandium

Depends on D192932

Differential Revision: https://phabricator.services.mozilla.com/D193704

build/docs/sccache-dist.rst

@@ -5,7 +5,12 @@ Distributed sccache (sccache-dist)
 ==================================
 
 `sccache <https://github.com/mozilla/sccache>`_ is a ccache-like tool written in
-Rust by Mozilla.
+Rust by Mozilla and many contributors.
+
+sccache-dist, its distributed variant, elevates this functionality by enabling
+the distribution and caching of Rust compilations across multiple machines.
+Please consider using sccache-dist when you have several machines
+compiling Firefox on the same network.
 
 The steps for setting up your machine as an sccache-dist server are detailed below.
 

docs/contributing/contribution_quickref.rst

@@ -97,6 +97,12 @@ Once the System is bootstrapped, run:
 which will check for dependencies and start the build.
 This will take a while; a few minutes to a few hours depending on your hardware.
 
+.. note::
+
+    If you build Firefox often, add `ac_add_options --with-ccache=sccache` to .mozconfig.
+    sccache will significantly speed up your builds by caching compilation results.
+    The Firefox build system will download sccache automatically.
+
 .. note::
 
     The default build is a compiled build with optimizations. Check out the

docs/setup/configuring_build_options.rst

@@ -219,6 +219,17 @@ supports `distributed compilation
 In order to enable ``sccache`` for Firefox builds, you can use
 ``ac_add_options --with-ccache=sccache``.
 
+From version 0.7.4, sccache local builds are using the ``preprocessor cache mode``
+by default. With a hot cache, it decreases the build time by a factor of 2 to 3
+compared the previous method. This feature works like the `direct mode in ccache
+<https://ccache.dev/manual/3.7.9.html#_the_direct_mode>`__,
+using a similar way to handle caching and dependencies.
+
+   .. note::
+
+      When using sccache, because of the operation on the files and storage,
+      the initial build of Firefox will be slower.
+
 Optimization
 ^^^^^^^^^^^^