Clarification in 6.6.3: if no isogeny exists, this method does not work

[kwantam]

I got a note from someone using the draft that they thought the wording in Section 6.6.3 could be clearer about the fact that the curve E' isogenous to E may not exist. Maybe one clarifying sentence at the end of the 3rd paragraph, roughly:

Note that it may not be possible to find a curve E' meeting these requirements, in which case this method does not apply.

Per @chris-wood we should be able to make this change (right?)

(note: stashing this conversation as an issue so that we don't lose track of it!)