[gentoo-dev] updated elass documentation syntax

[Mike Frysinger]

ive extended the eclass documentation format a little for variables.  you can 
now use the markers:

	@DEFAULT_UNSET
avoid overhead of adding useless constructs like:
	: ${FOO:=}
this way the awk script wont warn when it was unable to find a default value 
for the variable in question

	@REQUIRED
similar to the previous construct; this allows people to mark a variable so 
that ebuild writers know they must provide for this.  in the man page output, 
the variable will be marked with "(REQUIRED)".

	@INTERNAL
some people like to document all variables, even ones that ebuilds/users 
shouldnt be setting (i.e. internal state variables).  now you can do just that 
and the variable wont be included in the output.
-mike