RISC-V: align ISA extension Kconfig help text with each other

Other extensions only capitalise the first letter in the text visible
in Kconfig menus, and provide a short comment about the extension's
meaning. Do the same for Svnapot & Svpbmt.

The precedent for capitalisation in the Kconfig text was set by Zicbom
& sorta followed for Zicboz. The RVI styling used for multi-letter
extensions only capitalises the first letter, so do the same here.
If nothing else, my OCD likes it when the extensions follow a consistent
pattern.

While editing one of the lines, reformat the "spelling" of 64-bit.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230405-pucker-cogwheel-3a999a94a2f2@wendy


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>