Reindexing Python stuff with category "Python"

This closes #2355. It so happens you didn't need to special-code a new
category; as a matter of fact, there's a simple (if obscure) way of
doing it, which I have used. Also correcting some accidental indexing
somewhere else.

doc/Language/grammars.pod6

@@ -264,7 +264,8 @@ This comes in handy when you're already differentiating the proto regexes with
 the strings you're going to match, as using C«<sym>» token prevents repetition
 of those strings.
 
-=head3 X«"Always succeed" assertion|<?>»
+X<|<?>>
+=head3 "Always succeed" assertion
 
 The C«<?>» is the I<always succeed> assertion. When used as a grammar
 token, it can be used to trigger an Action class method. In the following

doc/Language/py-nutshell.pod6

@@ -263,11 +263,11 @@ Many of the other unicode operators work as you would expect
 (exponents, fractions, π), but every unicode operator
 or symbol that can be used in Perl 6 has an ASCII equivalent.
 
-=head2 Control flow
+=head2 X<Control flow|Python>
 
 Python has C<for> loops and C<while> loops:
 
-=for code :lang<python>
+=for code :lang<Python>
 for i in 1, 2:
     print i
 j = 1
@@ -346,7 +346,7 @@ Perl 6
     }
 
 
-=head2 Lambdas, functions and subroutines
+=head2 X<Lambdas, functions and subroutines|Python>
 
 Declaring a function (subroutine) with C<def> in Python is accomplished
 with C<sub> in Perl 6.
@@ -452,9 +452,10 @@ i.e. these are the same:
     my $power = { $^x ** $^y };
     my $power = -> $x, $y { $x ** $y };
 
-=head2 List comprehensions
+=head2 X<List comprehensions|Python>
 
-Postfix statement modifiers and blocks can be combined to make list comprehensions.
+Postfix statement modifiers and blocks can be combined to make list
+comprehensions.
 
 Python
 
@@ -491,9 +492,10 @@ becomes either of these:
 Using C<map> (which is just like Python's C<map>) and
 C<grep> (which is like Python's C<filter>) is an alternative.
 
-=head2 Classes and objects
+=head2 X<Classes and objects|Python>
 
-Here's an example from the Python L<docs|https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables>.
+Here's an example from the Python
+L<docs|https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables>.
 First, "instance variables", aka attributes in Perl 6:
 
 Python:
@@ -512,7 +514,7 @@ Perl 6:
 Constructors by default take named arguments in Perl 6,
 and use the method C<new>.
 
-Python
+Python:
 
 =for code :lang<python>
 d = Dog('Fido')
@@ -633,9 +635,7 @@ or
         ...
     }
 
-=head2 Decorators
-X<|Decorators (Python)>
-X<|@decorator (Python)>
+=head2 X<Decorators|Python>
 
 Decorators in Python are a way of wrapping a function
 in another one.  In Perl 6, this is done with C<wrap>.
@@ -685,15 +685,13 @@ An alternative would be to use a trait:
 
     world;
 
-=head2 Context managers
-X<|Context Managers (Python)>
-X<|with (Python)>
+=head2 X<Context managers|Python>
 
 Context managers in Python declare actions that happen when entering
 or exiting a scope.
 
-Here's a python context manager that prints the strings
-'hello', 'world', and 'bye':
+Here's a Python context manager that prints the strings
+'hello', 'world', and 'bye'.
 
 =begin code :lang<python>
 class hello:
@@ -706,7 +704,7 @@ with hello():
     print 'world'
 =end code
 
-For enter and exit events, passing a block as
+For "enter" and "exit" events, passing a block as
 an argument would be one option:
 
     sub hello(Block $b) {
@@ -728,8 +726,7 @@ run on entering or leaving a block.
         say 'world';
     }
 
-X<|input (Python)>
-=head2 Input
+=head2 X<C<input>|Python>
 
 In Python 3, the C<input> keyword is used to prompt the user: