document --pretty=tformat: option

This was introduced in 4da45bef, but never documented anywhere.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -124,3 +124,25 @@ The placeholders are:
 - '%m': left, right or boundary mark
 - '%n': newline
 - '%x00': print a byte from a hex code
+
+* 'tformat:'
++
+The 'tformat:' format works exactly like 'format:', except that it
+provides "terminator" semantics instead of "separator" semantics. In
+other words, each commit has the message terminator character (usually a
+newline) appended, rather than a separator placed between entries.
+This means that the final entry of a single-line format will be properly
+terminated with a new line, just as the "oneline" format does.
+For example:
++
+---------------------
+$ git log -2 --pretty=format:%h 4da45bef \
+  | perl -pe '$_ .= " -- NO NEWLINE\n" unless /\n/'
+4da45be
+7134973 -- NO NEWLINE
+
+$ git log -2 --pretty=tformat:%h 4da45bef \
+  | perl -pe '$_ .= " -- NO NEWLINE\n" unless /\n/'
+4da45be
+7134973
+---------------------