doc: interpret-trailers: commit to “trailer block” term

We chose to introduce the term “trailer block” into the documentation a
few commits ago.[1] It is used in the code though, so it is not a newly
invented term.

That term was useful to explain where the trailers are found (they
*trail* the message). But it is also useful here, where we explain how
trailers are added to existing messages, how trailer blocks are
found (beyond the simple case in the introduction), and how the end of
the message is found.

† 1: in commit “explain the format after the intro”

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/git-interpret-trailers.adoc

@@ -74,19 +74,21 @@ key: value
 This means that the trimmed _<key>_ and _<value>_ will be separated by
 "`:`{nbsp}" (one colon followed by one space).
 
-By default the new trailer will appear at the end of all the existing
-trailers. If there is no existing trailer, the new trailer will appear
-at the end of the input. A blank line will be added before the new
-trailer if there isn't one already.
-
-Existing trailers are extracted from the input by looking for
-a group of one or more lines that (i) is all trailers, or (ii) contains at
-least one Git-generated or user-configured trailer and consists of at
+By default the new trailer will appear at the end of the trailer block.
+A trailer block will be created with only that trailer if a trailer
+block does not already exist. Recall that a trailer block needs to be
+preceded by a blank line, so a blank line (specifically an empty line)
+will be inserted before the new trailer block in that case.
+
+Existing trailers are extracted from the input by looking for the
+trailer block. Concretely, that is a group of one or more lines that (i)
+is all trailers, or (ii) contains at least one Git-generated or
+user-configured trailer and consists of at
 least 25% trailers.
-The group must be preceded by one or more empty (or whitespace-only) lines.
-The group must either be at the end of the input or be the last
-non-whitespace lines before a line that starts with `---` (followed by a
-space or the end of the line).
+The trailer block is by definition at the end the the message. The end
+of the message in turn is either (i) at the end of the input, or (ii)
+the last non-whitespace lines before a line that starts with `---`
+(followed by a space or the end of the line).
 
 For convenience, a _<key-alias>_ can be configured to make using `--trailer`
 shorter to type on the command line. This can be configured using the