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.

doc/lispref/nonascii.texi

@@ -583,17 +583,17 @@ This function returns the value of @var{char}'s @var{propname} property.
      @result{} Nd
 @end group
 @group
-;; subscript 4
+;; U+2084 SUBSCRIPT FOUR
 (get-char-code-property ?\u2084 'digit-value)
      @result{} 4
 @end group
 @group
-;; one fifth
+;; U+2155 VULGAR FRACTION ONE FIFTH
 (get-char-code-property ?\u2155 'numeric-value)
      @result{} 0.2
 @end group
 @group
-;; Roman IV
+;; U+2163 ROMAN NUMERAL FOUR
 (get-char-code-property ?\u2163 'numeric-value)
      @result{} 4
 @end group