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

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>