[docgen] proc doc comments silently omitted after 1st runnableExamples

[timotheecour]

We can have multiple runnableExamples blocks per proc (and they both get run and shown in html) however the doc comments after the 1st runnableExamples block aren't exported in generated html docs; this makes it currently impossible to replace documentation using multiple .. code-block:: nim sections (or a single one but with doc comments after it) with runnableExamples while maintaining the same docs. The proposal is simple: make subsequent doc comments blocks part of proc documentation, until the 1st empty line or start of code implementation to separate proc doc comment from proc implementation.

when defined(case1):
  proc foo*()=
    ## proc doc comment part 1
    runnableExamples: echo "ok1"
    ## proc doc comment part 2 BUG: not in generated html
    runnableExamples: echo "ok2"
    ## proc doc comment part 3 BUG: not in generated html

    ## not part of proc doc comment
    echo "ok3"

when defined(case2):
  proc foo*()=
    ## proc doc comment part 1
    ##
    ## .. code-block:: nim
    ##   echo "ok1"
    ##
    ## proc doc comment part 2
    ##
    ## .. code-block:: nim
    ##   echo "ok2"
    ##
    ## proc doc comment part 3

    ## not part of proc doc comment
    echo "ok3"

case1:

case2:

Note: see [1] for rationale as to why runnableExamples are preferred over .. code-block
[1] runnableExamples should generally be preferred to .. code-block:: nim - Nim forum