Document pseudo-tags "null" and ".".
--- a/doc/hg.1.txt
+++ b/doc/hg.1.txt
@@ -62,6 +62,14 @@ SPECIFYING SINGLE REVISIONS
The reserved name "tip" is a special tag that always identifies
the most recent revision.
+ The reserved name "null" indicates the null revision. This is the
+ revision of an empty repository, and the parent of revision 0.
+
+ The reserved name "." indicates the working directory parent. If
+ no working directory is checked out, it is equivalent to null.
+ If an uncommitted merge is in progress, "." is the revision of
+ the first parent.
+
SPECIFYING MULTIPLE REVISIONS
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