AsciiDoc text replacement: non-escaped ' -- '

https://docs.asciidoctor.org/asciidoc/latest/syntax-quick-reference/#text-replacements
notes that text replacement may occur where

– is between two word characters, between a word character and a
line boundary, or flanked by spaces. The replacement:

—

– an em dash.

The ripgrep command below helped to identify two editions of the
FreeBSD Developers' Handbook in which an escape charater was required.

rg '.* -- .*' /usr/doc

Whilst here: full stops, and one sentence per line for the affected
subsection of the zh-tw translation of the book.

https://bugs.freebsd.org/269679
PR: 269679
Pull request: https://github.com/freebsd/freebsd-doc/pull/113