doc: improve documentation of %ifid et al

Clarify some of the semantics of %ifid, %ifnum, and %ifstr.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

Author: H. Peter Anvin (Intel)

doc/changes.src

@@ -9,8 +9,8 @@ since 2007.
 
 \S{cl-2.15.05} Version 2.15.05
 
-\b Correct \c{%ifid $} and \c{%ifid $$} being treated as true. See
-\k{iftyp}.
+\b Fix \c{%ifid $} and \c{%ifid $$} incorrectly being treated as
+true. See \k{iftyp}.
 
 \b Add \c{--reproducible} option to suppress NASM version numbers and
 timestamps in output files. See \k{opt-reproducible}.

doc/nasmdoc.src

@@ -3376,9 +3376,13 @@ string.
 
 The conditional assembly construct \c{%ifid}, taking one parameter
 (which may be blank), assembles the subsequent code if and only if
-the first token in the parameter exists and is an identifier.
-\c{%ifnum} works similarly, but tests for the token being a numeric
-constant; \c{%ifstr} tests for it being a string.
+\e{the first token} in the parameter exists and is an
+identifier. \c{$} and \c{$$} are \e{not} considered identifiers by
+\c{%ifid}.
+
+\c{%ifnum} works similarly, but tests for the token being an integer
+numeric constant (not an expression!) possibly preceeded by \c{+} or
+\c{-}; \c{%ifstr} tests for it being a quoted string.
 
 For example, the \c{writefile} macro defined in \k{mlmacgre} can be
 extended to take advantage of \c{%ifstr} in the following fashion: