git-prompt: document how in-progress operations affect the prompt

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -70,6 +70,10 @@
 # state symbols by setting GIT_PS1_STATESEPARATOR. The default separator
 # is SP.
 #
+# When there is an in-progress operation such as a merge, rebase,
+# revert, cherry-pick, or bisect, the prompt will include information
+# related to the operation, often in the form "|<OPERATION-NAME>".
+#
 # By default, __git_ps1 will compare HEAD to your SVN upstream if it can
 # find one, or @{upstream} otherwise.  Once you have set
 # GIT_PS1_SHOWUPSTREAM, you can override it on a per-repository basis by