Fix gcc_jit_context_get_last_error docs

Fix gcc_jit_context_get_last_error docs

gcc/jit/ChangeLog:
	* docs/topics/compilation.rst (gcc_jit_result_get_code): Fix typo.
	* docs/topics/contexts.rst (gcc_jit_context_get_last_error): The
	error buffer is only valid until the next call to the context.
	* libgccjit.h (gcc_jit_context_get_first_error): Reword the
	comment to omit mention of compiling.
	(gcc_jit_context_get_last_error): The error buffer is only valid
	until the next call to the context.

From-SVN: r220145