git.txt: correct stale 'GIT_EXTERNAL_DIFF' description

In fde97d8 (Update documentation to remove incorrect GIT_DIFF_OPTS
example., 2006-11-27), the description of the 'GIT_EXTERNAL_DIFF'
variable was moved from 'diff-format.txt' to 'git.txt', and the
documentation was updated to remove a 'diff(1)' invocation since Git did
not use an external diff program anymore by default.

However, the description of 'GIT_EXTERNAL_DIFF' still mentions "instead
of the diff invocation described above", which is confusing.

Correct that outdated sentence.

Also, link to git(1) in 'diff-generate-patch.txt' when GIT_DIFF_OPTS and
GIT_EXTERNAL_DIFF are mentioned, so that users can easily know what
these variables are about.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>

Documentation/diff-generate-patch.txt

@@ -10,7 +10,8 @@ linkgit:git-diff-tree[1], or
 linkgit:git-diff-files[1]
 with the `-p` option produces patch text.
 You can customize the creation of patch text via the
-`GIT_EXTERNAL_DIFF` and the `GIT_DIFF_OPTS` environment variables.
+`GIT_EXTERNAL_DIFF` and the `GIT_DIFF_OPTS` environment variables
+(see linkgit:git[1]).
 
 What the -p option produces is slightly different from the traditional
 diff format:

Documentation/git.txt

@@ -551,8 +551,9 @@ Git Diffs
 
 `GIT_EXTERNAL_DIFF`::
 	When the environment variable `GIT_EXTERNAL_DIFF` is set, the
-	program named by it is called, instead of the diff invocation
-	described above.  For a path that is added, removed, or modified,
+	program named by it is called to generate diffs, and Git
+	does not use its builtin diff machinery.
+	For a path that is added, removed, or modified,
 	`GIT_EXTERNAL_DIFF` is called with 7 parameters:
 
 	path old-file old-hex old-mode new-file new-hex new-mode