pretty format %(trailers) doc: avoid repetition

Change the documentation for the various %(trailers) options so it
isn't repeating part of the documentation for "only" about how boolean
values are handled. Instead, let's split the description of that into
general documentation at the top.

It then suffices to refer to it by listing the options as
"opt[=<BOOL>]". I'm also changing it to upper-case "[=<BOOL>]" from
"[=val]" for consistency with "<SEP>"

It took me a couple of readings to realize that these options were
referring back to the "only" option's treatment of boolean
values.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -252,7 +252,15 @@ endif::git-rev-list[]
 			  interpreted by
 			  linkgit:git-interpret-trailers[1]. The
 			  `trailers` string may be followed by a colon
-			  and zero or more comma-separated options:
+			  and zero or more comma-separated options.
+			  If any option is provided multiple times the
+			  last occurance wins.
++
+The boolean options accept an optional value `[=<BOOL>]`. The values
+`true`, `false`, `on`, `off` etc. are all accepted. See the "boolean"
+sub-section in "EXAMPLES" in linkgit:git-config[1]. If a boolean
+option is given with no value, it's enabled.
++
 ** 'key=<K>': only show trailers with specified key. Matching is done
    case-insensitively and trailing colon is optional. If option is
    given multiple times trailer lines matching any of the keys are
@@ -261,27 +269,21 @@ endif::git-rev-list[]
    desired it can be disabled with `only=false`.  E.g.,
    `%(trailers:key=Reviewed-by)` shows trailer lines with key
    `Reviewed-by`.
-** 'only[=val]': select whether non-trailer lines from the trailer
+** 'only[=<BOOL>]': select whether non-trailer lines from the trailer
-   block should be included. The `only` keyword may optionally be
+   block should be included.
-   followed by an equal sign and one of `true`, `on`, `yes` to omit or
-   `false`, `off`, `no` to show the non-trailer lines. If option is
-   given without value it is enabled. If given multiple times the last
-   value is used.
 ** 'separator=<SEP>': specify a separator inserted between trailer
    lines. When this option is not given each trailer line is
    terminated with a line feed character. The string SEP may contain
    the literal formatting codes described above. To use comma as
    separator one must use `%x2C` as it would otherwise be parsed as
-   next option. If separator option is given multiple times only the
+   next option. E.g., `%(trailers:key=Ticket,separator=%x2C )`
-   last one is used. E.g., `%(trailers:key=Ticket,separator=%x2C )`
    shows all trailer lines whose key is "Ticket" separated by a comma
    and a space.
-** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
+** 'unfold[=<BOOL>]': make it behave as if interpret-trailer's `--unfold`
-   option was given. In same way as to for `only` it can be followed
+   option was given. E.g.,
-   by an equal sign and explicit value. E.g.,
    `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
-** 'valueonly[=val]': skip over the key part of the trailer line and only
+** 'valueonly[=<BOOL>]': skip over the key part of the trailer line and only
-   show the value part. Also this optionally allows explicit value.
+   show the value part.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will