minor Steenrod algebra fixes
#16196: rebased to #16232
Trac #16196: dedent pasted sage prompts

Sage prompts with leading whitespace are not removed correctly:
{{{
sage:    1+1     # works
2
sage: sage: 1+1     # works
2
sage:    sage: 1+1     # indent + prompt = fail
  File "<ipython-input-34-360e106e9b2f>", line 1
    sage: Integer(1)+Integer(1)     # indent + prompt = fail
        ^
SyntaxError: invalid syntax
}}}
Possibly due to #16050

URL: http://trac.sagemath.org/16196
Reported by: vbraun
Ticket author(s): R. Andrew Ohana
Reviewer(s): John Palmieri
Trac #15251: minor Steenrod algebra fixes

The attached patch makes some minor changes to the Steenrod algebra
code:

- changes a few docstrings and comments
- one bug fix (to make `SteenrodAlgebra(2, 'wood')` raise a more
meaningful error) with doctest
- cache the function `arnonC_basis` for a speed improvement. Before:
{{{
sage: %time B = SteenrodAlgebra(basis='arnon_c').basis(20)
CPU times: user 2.44 s, sys: 0.17 s, total: 2.60 s
Wall time: 2.51 s
sage: %time B = SteenrodAlgebra(basis='arnon_c').basis(100) # takes a
loooong time
}}}
 After:
{{{
sage: %time B = SteenrodAlgebra(basis='arnon_c').basis(20)
CPU times: user 0.01 s, sys: 0.00 s, total: 0.01 s
Wall time: 0.01 s
sage: %time B = SteenrodAlgebra(basis='arnon_c').basis(100)
CPU times: user 0.57 s, sys: 0.10 s, total: 0.67 s
Wall time: 0.60 s
}}}

URL: http://trac.sagemath.org/15251
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Christian Nassau