trailer --parse docs: add explanation for its usefulness

For users who are skimming the docs to go straight to the individual
breakdown of each flag, it may not be clear why --parse is a convenience
alias (without them also looking at the other options that --parse turns
on). To save them the trouble of looking at the other options (and
computing what that would mean), describe a summary of the overall
effect.

Similarly update the area when we first mention --parse near the top of
the doc.

Signed-off-by: Linus Arver <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Linus Arver

Documentation/git-interpret-trailers.txt

@@ -31,7 +31,9 @@ the last two lines starting with "Signed-off-by" are trailers.
 
 This command reads commit messages from either the
 <file> arguments or the standard input if no <file> is specified.
-If `--parse` is specified, the output consists of the parsed trailers.
+If `--parse` is specified, the output consists of the parsed trailers
+coming from the input, without influencing them with any command line
+options or configuration variables.
 Otherwise, this command applies the arguments passed using the
 `--trailer` option, if any, to each input file. The result is emitted on the
 standard output.
@@ -158,7 +160,10 @@ OPTIONS
 
 --parse::
 	A convenience alias for `--only-trailers --only-input
-	--unfold`.
+	--unfold`. This makes it easier to only see the trailers coming from the
+	input without influencing them with any command line options or
+	configuration variables, while also making the output machine-friendly with
+	--unfold.
 
 --no-divider::
 	Do not treat `---` as the end of the commit message. Use this