doc: md.texi (Insn Splitting): Tweak wording for readability.

I needed to check what was allowed in a define_split, but
had problems understanding what was meant by "Splitting of
jump instruction into sequence that over by another jump
instruction".

	* doc/md.texi (Insn Splitting): Tweak wording for readability.

Co-Authored-By: Sandra Loosemore <sandra@codesourcery.com>