Change "secret seed" to "secret".

[BenWestgate]

To improve clarity and reduce book and BIP word counts let's change "secret seed" to "codex32-encoded master seed" and after introducing the concept, just "secret"?

"Seed" or "master seed" only for the secret's bytes data.

"Secret" for the string with index "s", "share" for non-"s" string.

Avoid adding adjectives that are redundant, oxymoronic, add jargon or blur this distinction.

[BenWestgate]

I don't know where the booklet is located, it's not in codex32/master.

[apoelstra]

I think it's in https://github.com/apoelstra/codex32/tree/kiara-complete but I'm uncertain. @roconnor-blockstream and I hope to get all the booklet stuff (maybe excluding the illustrations which are an overwhelming quantity of machine-generated postscript) cleaned up, reviewed, and merged into master during Q1.

[BenWestgate]

A few places in bitcoin/bips#2052 roconnor and I used the book's term "secret seed" mostly replacing the "secret share" oxymoron to keep a distinct term for interpolated seeds vs encoded ones (codex32-encoded master seed vs secret seed). If that leaves the book something else may be better.