Documentation: clarify whitespace rules for trailers

Commit e431956 (trailer: be stricter in parsing separators, 2016-11-02)
restricted whitespaces allowed by `git interpret-trailers` in the "token"
part of the trailers it reads. This commit didn't update the related
documentation in Documentation/git-interpret-trailers.txt though.

Also commit 60ef86a (trailer: support values folded to multiple lines,
2016-10-21) updated the documentation, but didn't make it clear how many
whitespace characters are allowed at the beginning of new lines in folded
values.

Let's fix both of these issues by rewriting the paragraph describing
what whitespaces are allowed by `git interpret-trailers` in the trailers
it reads.

Signed-off-by: Christian Couder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: chriscool

Documentation/git-interpret-trailers.txt

@@ -60,10 +60,12 @@ non-whitespace lines before a line that starts with '---' (followed by a
 space or the end of the line). Such three minus signs start the patch
 part of the message. See also `--no-divider` below.
 
-When reading trailers, there can be whitespaces after the
-token, the separator and the value. There can also be whitespaces
-inside the token and the value. The value may be split over multiple lines with
-each subsequent line starting with whitespace, like the "folding" in RFC 822.
+When reading trailers, there can be no whitespace before or inside the
+token, but any number of regular space and tab characters are allowed
+between the token and the separator. There can be whitespaces before,
+inside or after the value. The value may be split over multiple lines
+with each subsequent line starting with at least one whitespace, like
+the "folding" in RFC 822.
 
 Note that 'trailers' do not follow and are not intended to follow many
 rules for RFC 822 headers. For example they do not follow