doc: add note about pattern rooted/unrooted cases to "hgignore" and "patterns"

each help topics describe that patterns are "not rooted" and "rooted"
in themselves, but not describe about each other.

so, this may causes misunderstanding about "rooted"-ness of patterns.

mercurial/help/hgignore.txt

@@ -63,6 +63,10 b' the form ``*.c`` will match a file endin'
 and a regexp pattern of the form ``\.c$`` will do the same. To root a
 regexp pattern, start it with ``^``.
 
+.. note::
+  Patterns specified in other than ``.hgignore`` are always rooted.
+  Please see :hg:`help patterns` for details.
+
 Example
 -------
 

mercurial/help/patterns.txt

@@ -6,6 +6,10 b' patterns.'
 
 Alternate pattern notations must be specified explicitly.
 
+.. note::
+  Patterns specified in ``.hgignore`` are not rooted. Please see
+  :hg:`help hgignore` for details.
+
 To use a plain path name without any pattern matching, start it with
 ``path:``. These path names must completely match starting at the
 current repository root.