Make users aware about deprecation of functions or methods

# Problem description

I'm quite new to the library and its API. By accidentally looking at the source code, like c++/src/H5Location.cpp, I discovered that certain functions or methods are deprecated by wrapping them with:

https://github.com/HDFGroup/hdf5/blob/07c4360b4d4f8459571815d624522eb7e01b02f6/c%2B%2B/src/H5Location.cpp#L2301-L2312

or

https://github.com/HDFGroup/hdf5/blob/07c4360b4d4f8459571815d624522eb7e01b02f6/c%2B%2B/src/H5Location.h#L195-L213

Unfortunately, that fact is not visible in the generated doxygen documentation, nor visible during compilation.

`getObjTypeByIdx` is not even listed here: https://docs.hdfgroup.org/hdf5/develop/deprecated.html.

# Possible solution

1. I would like to see some hints in the documentation that function or methods are deprecated and what to use instead.
2. And maybe it would help to optionally mark those methods with `[[deprecated]]`, which is available since C++14.