DOC - partial rewrite of fused types doc

Ignore byte compiled elisp modules in the repository.

* When cython.git is included as a submodule in an
  Emacs configuration to load cython-mode.el and is
  byte-compiled, it generates an .elc file which causes
  the submodule to appear dirty unnecessarily in the parent
  git repository. Adding this rule will avoid dirty submodules.
Signed-off-by: Yesudeep Mangalapilly <yesudeep@google.com>

--HG--
extra : transplant_source : %28%90%91h.%EE%94%1AE%F5%A9%DA33J%D0%60h%F5t

After conversation with Mark F today. Change from pointers to scalars
for clarity.  Add quickstart section.  Explain case where the same fused
type in an argument list must be specialized to same type.

--HG--
extra : transplant_source : %9F%0B%FF%0F%16%0C%25%B9%FA7%D8%25%9EDpJSE%5E%BB

--HG--
extra : transplant_source : %AE%FF9%0ALO%CE%5E%C7%93%FA%B1d%E73%E8%CA%BE%DC%C8

--HG--
extra : transplant_source : %B2Q%A0%A7c%3B%EE%40%93%08%E1%C9-%83%88%BA%ACU%7D%8D

--HG--
extra : transplant_source : %D5V%9A%D2%DE%A7%A8%20%21%05%FFp%DEk%C4f%3Ct%0DT

--HG--
extra : transplant_source : J%FF%93%9C%96%02%E8m%0E%AD7V%A5%1C%CF%7D%89%F6%14J

--HG--
extra : transplant_source : %D9%C4%E5%B7%FC9%0D%60%80R%E2%87%82S%9D%9C%8Bd%C3%C5

--HG--
extra : transplant_source : m%879%8BW%7D%87%87%F5oYoA%ABl%88I%EF%92%FA

--HG--
extra : transplant_source : n%98%20%EE%A3N%1D%20%3F%88%2A%8B%FE%A0%01%97%9F%04%ED%23

--HG--
extra : transplant_source : fE%C2RT%7EHt%958%F4%257o%00%CB%3B%C3%B7%BA

fixed unused function warning about __pyx_clear_code_object_cache() when cleanup code is not being generated

libcpp.string supports s[0]= and s.at(0)=