TARGET_FUNCTION_ATTRIBUTE_INLINABLE_P docs are wrong

[DJ Delorie]

It says:

	This target hook returns @code{true} if it is ok to inline
	@var{fndecl} into the current function,

However, it is not called in the context of a current function - it is
called when each function is parsed, and the "inlinable" tag is stored
with the function's trees.  That means that we can no longer use the
current function's context to determine inlinability.

Unfortunately, we have an internal port where a function can be
inlined if it has the same attributes as the caller, but not
otherwise, and we can no longer determine this.