Clarify path prefix rewrite / redirect docs for HTTPPathModifier

[briantkennedy]

What happened:

The ReplacePrefixMatch docstring is a bit confusing since it doesn't provide a value for ReplacePrefixMatch in the example: ReplacePrefixMatch specifies the value with which to replace the prefix match of a request during a rewrite or redirect. For example, a request to "/foo/bar" with a prefix match of "/foo" would be modified to "/bar".

What you expected to happen:

The intro sentence should have the replacement prefix in the example:

For example, a request to "/foo/bar" with a prefix match of "/foo" and a replace prefix of "/xyz" would be modified to "/xyz/bar"

Clarifying trailing slash behavior would ideally be incorporate into a table of incoming request, prefix match value, replace prefix and output, eg:

Request Path	Prefix Match	Replace Prefix	Rewritten
/foo/bar	/foo	/xyz	/xyz/bar
/foo/bar	/foo	/xyz/	/xyz/bar
/foo/bar	/foo/	/xyz	/xyz/bar
/foo/bar	/foo/	/xyz/	/xyz/bar
/foo	/foo	/bar	/bar

While making the above table, I noticed that empty string in ReplacePrefixMatch can be problematic since it will have inconsistent behavior when the request path is identical to the prefix match.

Request Path	Prefix Match	Replace Prefix	Rewritten
/foo/bar	/foo	empty string	/bar
/foo	/foo	empty string	/ (this should technically be empty string, but that's not a valid path).