Backport 1.54 1.53 1.52 1.51 1.50 and 1.49:

Backport 1.54 1.53 1.52 1.51 1.50 and 1.49:

Try to improve the explanation of the "raise" statement and how its arguments
are interpreted.
This closes SF bug #532467.

Fix Typo.

Reword explanation of global statement since an undeclared global is a
free variable and is subject to those rules.

Note the sole case in which the ban on "from ... import *" within a
function is enforced.

Remove the following restriction:
    Names bound by import statements may not occur in global
    statements in the same scope.
Why not?

Note that it is illegal to delete a cell variable.
Note that deleteing an unbound local will raise a NameError.