Improve definition of linkwords

doc/org.texi

@@ -2930,9 +2930,10 @@ abbreviated link looks like this
 
 @noindent
 @vindex org-link-abbrev-alist
-where the tag is optional.  Such abbreviations are resolved according to
-the information in the variable @code{org-link-abbrev-alist} that
-relates the linkwords to replacement text.  Here is an example:
+where the tag is optional.  The @i{linkword} must be a word; letter, numbers,
+@samp{-}, and @samp{_} are allowed here.  Abbreviations are resolved
+according to the information in the variable @code{org-link-abbrev-alist}
+that relates the linkwords to replacement text.  Here is an example:
 
 @lisp
 @group