bsls_annotation: _CMP_CLANG and documentation fix

Explicitly add mention and support for 'BSLS_PLATFORM_CMP_CLANG'
instead of relying on 'BSLS_PLATFORM_CMP_GNU' to be set since Clang
supports the full set of GCC attributes.

Fix documentation to add _USED and _UNUSED to @MACROS list.

(The older GCC version checks < 4.3 are removed since BDE won't build with older compilers anyway)