Fixed parsing signatures of functions that return function pointers.

- Function name parsing is only done for scopes denoted with
BOOST_LOG_FUNCTION, scopes marked with BOOST_LOG_NAMED_SCOPE are output
as is.
- The parser has been rewritten to behave more correctly in case if
function name contains template parameters and/or returns a function
type.
- Operator functions are not supported by the parser.
- Docs corrections.