Update installation docs wrt PEP 517

[vytas7]

At the time of writing, our installation docs suggest using --no-build-isolation in order to cythonize the modules in the "old" way. Almost none of this is needed now with the recent Falcon releases fully supporting PEP 517.

The only exception we could probably document is explicitly disallowing cythonization by setting the FALCON_DISABLE_CYTHON envvar, and installing from source.