man: reword "string is extended into PCR"

man: reword "string is extended into PCR"

Originally, people used "extend a PCR with a measurement" or
"measure some blob into the PCR". In our docs those uses got merged
into "extend a string into a PCR". The meaning is clear to developers, but
it's a very jargony syntax that wouldn't make any sense to somebody who encounters
it the first time. Let's return to the more natural original phrasing.

Also, change various "this is supposed to act as" to "this acts as". If it
doesn't work, we have a bug to fix. This indirection is not useful.

Also, "boot path" → "phase path" and other smaller cleanups.