am: refer to format-patch in the documentation

There were two reasons we didn't do this.  As "git am" is designed
to grok e-mailed patches, not necessarily taken out of a Git
repostiory or even if it came from a Git repository not necessarily
produced with format-patch, we didn't want to single it out as the
"blessed" input producer to the command.  Also, in the original
workflow that "git am" was invented for, the user of "am" was
expected to be a different person than the users of "format-patch".

But this is a very safe change to make in 2023.  Thanks to the
effort by many contributors, Git ended up becoming a bit more
popular than we initially thought it would be, and "format-patch",
which took me a few weeks to pursuade Linus to take in 2005, seems
to have become the de-facto standard tool to produce patch e-mails.

Interestingly, the documentation for "git apply", which is listed in
SEE ALSO section of "git am" documentation, does mention "am" and
"format-patch" as two things that are related but different from
"apply" in an early part.

Suggested-by: Kai Grossjohann <kai.grossjohann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -24,7 +24,9 @@ DESCRIPTION
 -----------
 Splits mail messages in a mailbox into commit log message,
 authorship information and patches, and applies them to the
-current branch.
+current branch. You could think of it as a reverse operation
+of linkgit:git-format-patch[1] run on a branch with a straight
+history without merges.
 
 OPTIONS
 -------
@@ -273,7 +275,8 @@ include::config/am.txt[]
 
 SEE ALSO
 --------
-linkgit:git-apply[1].
+linkgit:git-apply[1],
+linkgit:git-format-patch[1].
 
 GIT
 ---