more patches for linking docs

[Matt Dew]

A couple quick patches to allow some links in the shapelib doc.

Also,
Up until now all, and including these patches, the id attributes have been 
all lowercase rather than matching the case of the text their for,
IE: MapError:  id='maperror' instead of 'id='MapError'.   xml is case sensitive so 
everything that links to these must match the case.

I thought this would be more consistent however after thinking about it and seeing someone 
comment on the irc about a function and a declared constant being identical except 
for capitolization, I think now  I made the wrong choice.

Thoughts?  Should I stop making the id attributes all lowercase and
instead make them match whatever item they are for? That will require going back
and updating all the previous ids at some point.

marcoz