Clarify that fragment identifier parsing algorithm is specific to HTTPS URLs?

[msporny]

From issue #126 (comment), @iherman noted that it would be good for us to

• add a sentence to the new section noting that the referred algorithm (retrieve verification method) is one of the possible ways to get to the controller document, and that application areas and/or implementations may decide to choose a different approach
• add a similar note to the algorithmic part emphasizing that manipulating the URL and its fragment part is an approach that VC-s and DID-s may use? usually use? (I do not know which), but this is not the only way to do this.

[TallTed]

for the first bullet...
s/ways to get to the controller document/ways to get to the controlled identifier document/