Use anon register in DoxygenUtility/PythonDocUtil.

vim/scripts/DoxygenUtility/autoload/(PythonDocUtil, DoxygenUtility).vim
	-Replace use of the `a` register with the anonymous register for
	value-capturing in either file's `s:CaptureOutputInRegister()`,
	because using a register beside it doesn't change anything (as
	the anonymous register mirrors the contents of the register last
	used).