[Headers][X86] Make brief descriptions briefer (#82422)

In Sony's document processing, the first "paragraph" of the description
is duplicated into a Brief Description section. Add paragraph breaks to make
those brief descriptions less verbose.

In fmaintrin.h we were including the \code blocks, which are
inappropriate for a brief description. While I was in there, change
\code to \code{.operation} which the Intel document processing wants.