document python_requires_extend attribute

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from conan-io/hooks#124 (comment)
it appears that attribute python_requires_extend is not documented on the attributes reference section. meanwhile, it's mentioned at least twice in the docs:

• https://docs.conan.io/en/latest/reference/conanfile/methods.html?highlight=python_requires_extend
• https://docs.conan.io/en/latest/extending/python_requires.html?highlight=python_requires_extend

the documentation block suggested by @jgsogo :

python_requires_extend is the way to inherit from a base class inside a python requires. 
Conan injects this class as a base class to the conanfile in runtime. 
Why? We need to be able to load the python file with the recipe and 
the python requires information before resolving the classes/methods from 
python_requires (if we use Python inheritance we need to retrieve 
python_requires first and the interpreter will use the base class). This let Conan 
control the python_requires.

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block has been added