Add support for documenting DeclareSynonym uses

[gwhitney]

If you write something like

#! @Description This category is defined as the intersection of two others.
DeclareSynonym( "IsMyIntersectionCategory", IsMyFirstCategory and IsMySecondCategory );

no documentation is produced. Looking at the code, this is not surprising; there is no code to recognize DeclareSynonym.

Note that a fix to this would also require a command to set the "type" of a declaration (Func/Filt/Meth/Constr/etc.) since there is no way to infer from a DeclareSynonym the type of the entity being declared, since it is simply the same as whatever it's being made a synonym of.

I have proposed code that adds support for DeclareSynonym, among several changes, which I will post soon.

[nathancarter]

I second this feature request. In some circumstances, users actually get error messages. See frankluebeck/GAPDoc#40 that actually belongs here. (But I won't file it here because it would be a duplicate of this issue.)

[fingolfin]

Note that ideally there should then also be support for DeclareSynonymAttr.

[wilfwilson]

I would also really like this, yes for both DeclareSynonym and DeclareSynonymAttr. I will have a look at #179 and #216 soon, and see if I can make sense of it and extract any useful bits/help to push it forward.