Clarify or replace a few \u escapes.

Clarify or replace a few \u escapes.

* doc/lispref/nonascii.texi (Character Properties)
More-detailed commentary for \u escapes.
* lisp/progmodes/python.el (python--prettify-symbols-alist):
* lisp/replace.el (query-replace-from-to-separator):
* lisp/textmodes/rst.el (rst-bullets, rst-re-alist-def)
(rst-mode-syntax-table):
* lisp/whitespace.el (whitespace-display-mappings):
Prefer actual character to \u escape when this makes the code
easier to follow in the usual case where Unicode chars can be
displayed.