FILTER_VALIDATE_URL: document some limitations in the validation

[DanielEScherzer]

This filter accepts a lot of things that might be unexpected, which led to some Mythos reports. While the limited validation isn't a security issue in PHP itself, it is worth calling out for anyone using FILTER_VALIDATE_URL for security-sensitive validation in applications.

[jordikroon]

LGTM. Though not related to this change, we should probably split this manual page into sub pages:

• Input Constants
• Generic Filter Flags
• Sanitization Filter Flags
• Validation Filters
• User Defined Filter

Kind of writing this for myself, but if anyone wants to take the job. Feel free.

[DanielEScherzer]

I'll leave this open for a bit before merging in case there are any objections