Extend documentation with implicitly disabled checkers under --enable-all

[bruntib]

--enable-all flag doesn't enable all checkers. Alpha, debug and some style checkers remain disabled. The full list of these should be documented in this section: https://github.com/Ericsson/codechecker/blob/master/docs/analyzer/user_guide.md#enable-all

codechecker/analyzer/codechecker_analyzer/analyzers/config_handler.py

Lines 175 to 178 in ed16b5d

	disabled_groups = ["alpha.", "debug.", "osx.", "abseil-", "android-",
	"darwin-", "objc-", "cppcoreguidelines-",
	"fuchsia.", "fuchsia-", "hicpp-", "llvm-",
	"llvmlibc-", "google-", "zircon-"]