clarify glInvalidateFramebuffer page for z/s

[zmike]

The core spec says:

Note that if a specified attachment has base internal format DEPTH_STENCIL but the attachments list
does not include DEPTH_STENCIL_ATTACHMENT or both DEPTH_ATTACHMENT
and STENCIL_ATTACHMENT, then only the specified portion of every pixel in the
subregion of pixels of the DEPTH_STENCIL buffer may be invalidated, and the
other portion must be preserved.

This should probably be in the reference page