Clarify regex usage for constr in documentation

[lmcnearney]

It might be helpful to clarify that the constr type validator's regex argument uses re.match() which only requires a partial match on the string. This personally bit me as I assumed the ^ and $ weren't required and put a length restriction in the expression but the field validated as the start of the string matched.

Updating the example in the documentation would probably cover it:

regex_str: constr(regex='^apple (pie|tart|sandwich)$')

Alternatively, it could use re.fullmatch() but that would change the behavior of the validator.

[samuelcolvin]

Thanks for reporting, perhaps fullmatch would have been a good idea, but we can't really change the behaviour until v2 (if you think that's a good idea, I guess create a clear new issue).

In terms of documentation, I completely, PR very welcome.