Clarify that percent-encoding often does not roundtrip

[annevk]

Also add some <div algorithm> wrappers to this section.

Closes #523.

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cc @karwa

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[domenic]

I'm not sure I understand the connection to the original issue. (Probably, I don't understand the original issue.)

The implicit statement here seems to be that because of the URL parser using neither percent-encode set, there are strings where the URL serialization of the URL parsing of "x" is not "x". If that's true, adding an example would be great.

But #523 seems to be talking fairly specifically about the username component, or about some sort of component-wise operation. I can't seem to produce any such strings using the live URL viewer and any of the examples therein.

[annevk]

Yeah, the statement was somewhat confusing. It's more that if you want to put data in a URL that has a % in it, you better percent-encode it first.