trailer --unfold help: prefer "reformat" over "join"

The phrase "join whitespace-continued values" requires some additional
context. For example, "whitespace" means newlines (not just space
characters), and "join" means to join only the multiple lines together
for a single trailer (and not that we are joining multiple trailers
together). That is, "join" means to convert

    token: This is a very long value, with spaces and
      newlines in it.

to

    token: This is a very long value, with spaces and newlines in it.

and does not mean to convert

    token: value1
    token: value2

to

    token: value1 value2.

Update the help text to resolve the above ambiguity. While we're add it,
update the docs to use similar language as the change in the help text.

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

Documentation/git-interpret-trailers.txt

@@ -155,8 +155,8 @@ OPTIONS
 	variables.
 
 --unfold::
-	Remove any whitespace-continuation in trailers, so that each
-	trailer appears on a line by itself with its full content.
+	If a trailer has a value that runs over multiple lines (aka "folded"),
+	reformat the value into a single line.
 
 --parse::
 	A convenience alias for `--only-trailers --only-input

builtin/interpret-trailers.c

@@ -106,7 +106,7 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 
 		OPT_BOOL(0, "only-trailers", &opts.only_trailers, N_("output only the trailers")),
 		OPT_BOOL(0, "only-input", &opts.only_input, N_("do not apply trailer.* configuration variables")),
-		OPT_BOOL(0, "unfold", &opts.unfold, N_("join whitespace-continued values")),
+		OPT_BOOL(0, "unfold", &opts.unfold, N_("reformat multiline trailer values as single-line values")),
 		OPT_CALLBACK_F(0, "parse", &opts, NULL, N_("alias for --only-trailers --only-input --unfold"),
 			PARSE_OPT_NOARG | PARSE_OPT_NONEG, parse_opt_parse),
 		OPT_BOOL(0, "no-divider", &opts.no_divider, N_("do not treat \"---\" as the end of input")),