Documentation: Clarified current role of ITEM special property

doc/org.texi (section on Special Properties) Clarified that ITEM refers
to the headline of the entry not the whole entry text.
(section on Matching Tags and Properties) Clarified that matching on
the value of ITEM special property is not currently supported,
and added a footnote about a partial workaround.
Also added missing -if suffix to references to org-agenda-skip-entry-if.

TINYCHANGE

doc/org.texi

@@ -4953,7 +4953,7 @@ TIMESTAMP_IA @r{The first inactive timestamp in the entry.}
 CLOCKSUM     @r{The sum of CLOCK intervals in the subtree.  @code{org-clock-sum}}
              @r{must be run first to compute the values in the current buffer.}
 BLOCKED      @r{"t" if task is currently blocked by children or siblings}
-ITEM         @r{The content of the entry.}
+ITEM         @r{The headline of the entry.}
 FILE         @r{The filename the entry is located in.}
 @end example
 
@@ -7536,6 +7536,9 @@ So a search @samp{+LEVEL=3+boss-TODO="DONE"} lists all level three headlines
 that have the tag @samp{boss} and are @emph{not} marked with the TODO keyword
 DONE.  In buffers with @code{org-odd-levels-only} set, @samp{LEVEL} does not
 count the number of stars, but @samp{LEVEL=2} will correspond to 3 stars etc.
+The ITEM special property cannot currently be used in tags/property
+searches@footnote{But @pxref{x-agenda-skip-entry-regexp,
+,skipping entries based on regexp}.}.
 
 Here are more examples:
 @table @samp
@@ -15876,9 +15879,10 @@ Skip current entry if the TODO keyword is TODO or WAITING.
 Skip current entry if the TODO keyword marks a DONE state.
 @item (org-agenda-skip-entry-if 'timestamp)
 Skip current entry if it has any timestamp, may also be deadline or scheduled.
-@item (org-agenda-skip-entry 'regexp "regular expression")
+@anchor{x-agenda-skip-entry-regexp}
+@item (org-agenda-skip-entry-if 'regexp "regular expression")
 Skip current entry if the regular expression matches in the entry.
-@item (org-agenda-skip-entry 'notregexp "regular expression")
+@item (org-agenda-skip-entry-if 'notregexp "regular expression")
 Skip current entry unless the regular expression matches.
 @item (org-agenda-skip-subtree-if 'regexp "regular expression")
 Same as above, but check and skip the entire subtree.