Cleanup doc of LinearCode's functions

[Johan Rosenkilde]

Go through the docstrings of the linear_code.py file, as well as all the functions. These should be cleaned up and rewritten.

• Module doc-string is way too long. Should just be index-like. See other module docs.
• Module doc-string of "Linear Code constructions" is insane. It should also refer to codes.<tab>.
• Clean index.rst file.
• "Binary self-dual codes"???
• The file binary_code.pyx is not present in the reference manual?
• Module doc-string for codecan is bad English and probably too long.
• "Bounds for parameters of codes" is insane!