[Lazarus] Bugs in LCL documentation

[Hans-Peter Diettrich]

I've just found and fixed an bug in the file search of build_lcl_docs, 
which prevented proper search of subdirectories. This made e.g. the 
wrong error message about forms\forms.pp disappear, now instead new 
problems with missing files are reported, mostly addressing the 
interfaces\ (widgetsets) directories.

Now we can start to fix the hints and errors, reported by fpdocs (until 
it crashes with an AV). A couple of entries read like

Warning: Target ID of <link> is unknown: 
"#LCL.StdCtrls.TCustomButton.Destroy"

where the links often lead to inexistent or protected class members, for 
which no documentation is created (by default). Should we simply ignore 
such hints, or should the links go to the public properties instead of 
their protected getters/setters?

DoDi