target.def (custom_function_descriptors): Improve documentation.

target.def (custom_function_descriptors): Improve documentation.

2018-08-30  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/
	* target.def (custom_function_descriptors): Improve documentation.
	* doc/tm.texi.in (Trampolines): Expand discussion of function
	descriptors and move TARGET_CUSTOM_FUNCTION_DESCRIPTORS to the
	beginning of the section.
	* doc/tm.texi: Regenerated.

From-SVN: r263999