SpecifierSet.contains's installed kwarg not following PEP 440

[gsnedders]

#505 / #515 added a installed kwarg to SpecifierSet.contains.

PER 440 says:

Pre-releases of any kind, including developmental releases, are implicitly excluded from all version specifiers, unless they are already present on the system, explicitly requested by the user, or if the only available version that satisfies the version specifier is a pre-release.

This implies that SpecifierSet(">=1,<2").contains('1.5a1', installed=True) should be True. However, this returns False.

(The docstring should also be updated to document the installed kwarg!)