doc: lei-mail-formats: add "eml" and expand on git things

While "eml" is not an output format, it seems worthy
to document, here, since users are likely to have experience
with *.patch files from "git format-patch".

1 files changed, 19 insertions, 1 deletions

diff --git a/Documentation/lei-mail-formats.pod b/Documentation/lei-mail-formats.pod
index da723d2b..3c37c880 100644
--- a/Documentation/lei-mail-formats.pod
+++ b/Documentation/lei-mail-formats.pod
@@ -51,7 +51,9 @@ with any number of C<E<gt>> characters and is thus fully
 reversible.
 
 This format is emitted by L<PublicInbox::WWW(3pm)> with gzip.
-It is supported by L<git-am(1)> since git 2.10.
+Since git 2.10, C<git am --patch-format=mboxrd> reads this
+format.  C<git log> and C<git format-patch --stdout> can also
+generate this format with the C<--pretty=mboxrd> switch.
 
 As with uncompressed L</mboxo>, uncompressed mboxrd are vulnerable
 to undetectable truncation.
@@ -96,6 +98,22 @@ Depending on the IMAP server software and configuration, IMAP
 servers may use any (or combination) of the aforementioned
 formats or a non-standard database backend.
 
+=head1 eml
+
+A single raw message file.  C<eml> is not an output format for lei,
+but accepted by as an C<--input-format> (C<-F>) for read-only
+commands such as L<lei-tag(1)> and L<lei-import(1)>.
+
+Since C<eml> is the suffix for the C<message/rfc822> MIME type
+(according to the C<mime.types> file), lei will infer the type
+based on the C<.eml> suffix if C<--input-format> is unspecified
+
+C<.patch>-suffixed files generated by L<git-format-patch(1)>
+(without C<--stdout>) are C<eml> files with the addition of an
+mbox C<From > header.  L<lei(1)> removes C<From > lines to treat
+them as C<eml> when reading these for compatibility with
+C<git-am(1)> and similar tools.
+
 =head1 COPYRIGHT
 
 Copyright 2021 all contributors L<mailto:meta@public-inbox.org>