Copyedits only; no technical content added. Changes:
- subject-verb agreement: "idea of registers were introduced" -> "was"
- "operate as the speed" -> "operate at the speed"
- fixed ungrammatical "typing as apart from X, Y is Z" (now "typing:
apart from X, Y is Z")
- comma after "x30" in list; replaced misleading claim "x31 is also
not available" with "x31 has a special role (explained later)".
x31 is not unavailable -- it encodes SP or XZR depending on
instruction context, which the earlier phrasing obscured
- "hundreds of instructions each potentially has" -> "each potentially
with" (agreement)
- "an mnemonic" -> "a mnemonic" (/n/ is a consonant sound)
- "a concrete examples" singular/plural typo
- de-duplicated back-to-back "For example" in Mixing Register Types
Rejected: leaving the "x31 is not available" line alone -- it's an
intro chapter, but "not available" reads as "doesn't exist", which
will contradict later chapters that use SP and XZR. Kept other
simplifications (informal "that big of" phrasing, stylistic comma use)
untouched.
