doc: improve ccache explanation

ccache improves the build speed significantly only when the branch was
built before. Building fresh branches with major changes are not
benefited from ccache.

PR-URL: #50133
Fixes: #49967
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

BUILDING.md

@@ -543,7 +543,8 @@ export CC="ccache cc"    # add to ~/.zshrc or other shell config file
 export CXX="ccache c++"  # add to ~/.zshrc or other shell config file
 ```
 
-This will allow for near-instantaneous rebuilds even when switching branches.
+This will allow for near-instantaneous rebuilds when switching branches back
+and forth that were built with cache.
 
 When modifying only the JS layer in `lib`, it is possible to externally load it
 without modifying the executable: