retrieve function documentation correctly
fix docstring extraction for "fancy" docs

The docstrings for yas-expand and yas-expand-from-keymap are generated
on the fly, depending on how they are called ("context"). Add a context
argument to the synthesizing functions and pass it nil when extracting
docstrings for the manual.
check function doc before variable doc

This means functions shadow variables with the same name, but at least
that's better than listing functions with the documentation from the
variable! (That happened because the call to yas--document-symbols in
snippet-reference.org was prefering function values to variables, while
the yas--document-symbol was prefering variable doc to function doc).
Merge pull request #436 from npostavs/autodoc-1,2

Fix points 1 and 2 of #434