Improve doc for emacs-source-directory (Bug#36527).

Author: eggert

INSTALL

@@ -219,8 +219,8 @@ DEBUGGING AN INSTALLED EMACS
 
 * Installed Emacs source code
 
-Emacs installs a compressed copy of much of its source code, to make
-it easy for users to read the source code of Emacs via commands like
+Emacs typically installs a compressed copy of much of its source code,
+to make it easy for users to read Emacs source code via commands like
 M-x describe-function (C-h f) to display the definition of a function.
 This compressed copy ordinarily includes both the Elisp source code
 that Emacs is mostly written in, as well as the C source code for the

etc/NEWS

@@ -148,8 +148,9 @@ called when the function object is garbage-collected.  Use
 ** 'parse-time-string' can now parse ISO 8601 format strings,
 such as "2020-01-15T16:12:21-08:00".
 
-** The new variable 'emacs-source-directory' gives the Emacs source
-code location.
+** The new variable 'emacs-source-directory' gives the location of
+a copy of the Emacs source code, which is now typically installed as a
+set of compressed source-code files.
 
 
 * Changes in Emacs 28.1 on Non-Free Operating Systems