(doc) Circle C++ shader restriction list

[FROL256]

It would be nice to have a doc page with list of features which should not be used at all (due to some fundamental restrictions of SPIR-V) or should not be used at least for a while (due to they are not implemented yet). For example, like "OpenCL C Restrictions" section here https://github.com/google/clspv/blob/master/docs/OpenCLCOnVulkan.md

Currently there is a section in documentation called "Approaching Circle shaders from C++" which is ok, but ... user should guess about concrete features (and btw it is not evident that this section is about restrictions). I think that just stupid "do this, don't do that" with some examples which features are strictly prohibited would be enough and helps.

It would also be great to place this in different .md file because main page became large )

[seanbaxter]

Not a bad idea.

Also, I should split the document into an examples page and a reference page before it gets much longer.