[cpp.pre] Clarify that the rule for conditionally-supported-declarations

resolves a parse ambiguity rather than determining program validity.

Author: zygoloid

source/preprocessor.tex

@@ -211,8 +211,9 @@
 \pnum
 A sequence of preprocessing tokens is only a \grammarterm{text-line}
 if it does not begin with a directive-introducing token.
-A \grammarterm{conditionally-supported-directive} shall not begin
-with any of the directive names appearing after a \tcode{\#} in the syntax.
+A sequence of preprocessing tokens is only a \grammarterm{conditionally-supported-directive}
+if it does not begin with any of the directive names
+appearing after a \tcode{\#} in the syntax.
 A \grammarterm{conditionally-supported-directive} is
 conditionally-supported with
 \impldef{additional supported forms of preprocessing directive}