Add footnote on how the hyperlink text is searched in Org files

* org.texi (External links): Add footnote on how the behavior of the
text search in Org files are controled by the variable
org-link-search-must-match-exact-headline.

This change is necessary in order to avoid misleading the user as
regards of the behavior of the search, in particular because the
default value of org-link-search-must-match-exact-headline is
'query-to-create.

TINYCHANGE

doc/org.texi

@@ -3172,7 +3172,13 @@ file:/myself@@some.where:papers/last.pdf   @r{file, path on remote machine}
 /myself@@some.where:papers/last.pdf        @r{same as above}
 file:sometextfile::NNN                    @r{file with line number to jump to}
 file:projects.org                         @r{another Org file}
-file:projects.org::some words             @r{text search in Org file}
+file:projects.org::some words             @r{text search in Org file}@footnote{
+The actual behavior of the search will depend on the value of
+the variable @code{org-link-search-must-match-exact-headline}.  If its value
+is nil, then a fuzzy text search will be done.  If it is t, then only the
+exact headline will be matched.  If the value is @code{'query-to-create},
+then an exact headline will be searched; if it is not found, then the user
+will be queried to create it.}
 file:projects.org::*task title            @r{heading search in Org file}
 docview:papers/last.pdf::NNN              @r{open file in doc-view mode at page NNN}
 id:B7423F4D-2E8A-471B-8810-C40F074717E9   @r{Link to heading by ID}