doc: clarify special schemes
In url.md, describe protocol schemes as "deemed special by the [standard]" rather than "considered to be special by the [standard]". The use of "considered to be" suggests that they might not really be. While "deemed" is a synonym for the most part, it connotes less doubt.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines