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When using pybind::options to disable function signatures, user-defined
docstrings only get appended if they exist, but newlines were getting
appended unconditionally, so the docstring could end up with blank lines
(depending on which overloads, in particular, provided docstrings).

This commit suppresses the empty lines by only adding newlines for
overloads when needed.

Added the docstring_options class, which gives global control over the generation of docstrings and function signatures.