doc: trailer: drop "commit message part" phrasing

doc: trailer: drop "commit message part" phrasing

The command can take inputs that are either just a commit message, or
an email-like output such as git-format-patch which includes a commit
message, "---" divider, and patch part. The existing explanation blends
these two inputs together in the first sentence

    This command reads some patches or commit messages

which then necessitates using the "commit message part" phrasing (as
opposed to just "commit message") because the input is ambiguous per the
above definition.

This change separates the two input types and explains them separately,
and so there is no longer a need to use the "commit message part"
phrase.

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>