Improvements to documentation of gcc_jit_context_release

Improvements to documentation of gcc_jit_context_release

gcc/jit/ChangeLog:
	* docs/examples/tut02-square.c (main): Release the context
	earlier, to show that this is possible.  Update error-handling
	to avoid a double-release of the context, and to avoid
	releasing a NULL result.
	* docs/intro/tutorial02.rst: Discuss gcc_jit_context_release.
	* docs/topics/functions.rst (GCC_JIT_FUNCTION_EXPORTED): Emphasize
	* docs/topics/results.rst (gcc_jit_result): Mention that this
	controls the lifetimes of machine code functions.
	(gcc_jit_result_get_code): Spell out the requirements for this
	to succeed, and the lifetime of the result.
	(gcc_jit_result_release): Mention that this invalidates any code
	that was obtained from the result.
	* docs/_build/texinfo/libgccjit.texi: Regenerate.

From-SVN: r218245