share/examples: clarify BOOT_COMCONSOLE_SPEED

Give an example of why one may want to override BOOT_COMCONSOLE_SPEED so
that it's clear why the default (in a comment) and the commented-out
example setting have different values.

Reported by: otis
Fixes: 4722ceb7d53e ("Use 115200 bps by default for serial communication")