docgen doesn't create links for certain symbols

[timotheecour]

I really like the new feature introduced by @Araq in 13d667e (docgen: produce links for proc names).

it doesn't work in some cases eg:

in https://nim-lang.github.io/Nim/osproc.html

• Process = ref ProcessObj => no link for ProcessObj
• no link for enum elements eg: poStdErrToStdOut here:

proc startCmd(command: string;
             options: set[ProcessOption] = {poStdErrToStdOut, poUsePath}): Process

• no links for symbols inside a runnableExamples block (this one is more controversial but would be nice IMO):
  (non runnableExamples code block may be impossible as it's not guaranteed to be runnable, but runnableExamples should be)

var t = {"name": "John", "city": "Monaco"}.newStringTable
doAssert "${name} lives in ${city}" % t == "John lives in Monaco"

[EDIT]

• no clickable links for constants, even though apparently they have anchor, eg:
  https://nim-lang.github.io/Nim/os.html#ExeExt has anchor but no underline clickable links for it:
  

[narimiran]

Two and a half raised points are fixed (with half being: there's no link for enum elements, but in the example options: set[ProcessOption] = {poStdErrToStdOut, poUsePath}: ProcessOption is clickable and there you can see each enum element).

The remaining point (links in runnableExamples) is, as stated, "more controversial but would be nice", and I don't see enough ROI in it. Therefore, I consider this issue to be fixed.