GCC -Wunused-parameter warnings and misspelling

I built Boost 1.72 with GCC 7.5 with added -Wextra in cxxflags. Problems are still in current code:

libs/python/src/object/function_doc_signature.cpp:108:118: warning: unused parameter ‘cpp_types’ [-Wunused-parameter]
libs/python/src/object/function_doc_signature.cpp:108:99: warning: unused parameter ‘n_overloads’ [-Wunused-parameter]

Misspelling: libs/python/src/object/function_doc_signature.cpp:37: shold

Open libs/python/src/object/function_doc_signature.cpp around lines 37 and 108 and inspect the spelling and unused-parameter warnings. Rebuild Boost.Python with GCC and -Wextra to verify the reported warnings are resolved and the misspelling is corrected.

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