Don't generate pawndoc for unimpemented functions

[Y-Less]

Issue description:

A forwarded function with no implementation will still be placed in the XML file with -r. Since these functions don't actually exist and can't be called, I don't think they should appear in the documentation. Only implemented functions (including those that are unused) should.

Minimal complete verifiable example (MCVE):

forward Unused();

main() {}

Output:

<?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet href="file:///C|/Users/Alex/Documents/SA-MP/pawno/xml/pawndoc.xsl" type="text/xsl"?>
<doc source="C:\Users\Alex\Documents\SA-MP\gamemodes\unused.pwn">
	<assembly>
		<name>unused.pwn</name>
	</assembly>
	<members>

		<!-- enumerations -->

		<!-- constants -->

		<!-- variables -->
		<member name="F:__date">
		</member>
		<member name="F:__file">
		</member>
		<member name="F:__time">
		</member>

		<!-- functions -->
		<member name="M:Unused" syntax="Unused()">
			<stacksize value="1"/>
		</member>
		<member name="M:main" syntax="main()">
			<attribute name="entry"/>
			<stacksize value="1"/>
		</member>

	</members>
</doc>

Workspace Information:

• Compiler version:
• Command line arguments provided (or sampctl version):
• Operating System:

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