Clarify the expand-file-name doc string

* src/fileio.c (Fexpand_file_name): Clarify that "~" in NAME is
expanded, and not just "~/".  Also clarify that ~USER is not
expanded if USER doesn't exist (bug#36490).

src/fileio.c

@@ -745,16 +745,22 @@ a directory's file name are accepted.  If DEFAULT-DIRECTORY is nil or
 missing, the current buffer's value of `default-directory' is used.
 NAME should be a string that is a valid file name for the underlying
 filesystem.
-File name components that are `.' are removed, and
-so are file name components followed by `..', along with the `..' itself;
-note that these simplifications are done without checking the resulting
-file names in the file system.
-Multiple consecutive slashes are collapsed into a single slash,
-except at the beginning of the file name when they are significant (e.g.,
-UNC file names on MS-Windows.)
-An initial `~/' expands to your home directory.
-An initial `~USER/' expands to USER's home directory.
-See also the function `substitute-in-file-name'.
+
+File name components that are `.' are removed, and so are file name
+components followed by `..', along with the `..' itself; note that
+these simplifications are done without checking the resulting file
+names in the file system.
+
+Multiple consecutive slashes are collapsed into a single slash, except
+at the beginning of the file name when they are significant (e.g., UNC
+file names on MS-Windows.)
+
+An initial \"~\" in NAME expands to your home directory.
+
+An initial \"~USER\" in NAME expands to USER's home directory.  If
+USER doesn't exist, \"~USER\" is not expanded.
+
+To do other file name substitutions, see `substitute-in-file-name'.
 
 For technical reasons, this function can return correct but
 non-intuitive results for the root directory; for instance,