Remove references to DocBook markup

[daobrien]

https://stylepedia.net/style/6.0/#view-edit-files

This section uses a couple of examples of DocBook markup. Need to make them more generic.

[julian-cable]

@daobrien Also in the same section, what does "here" documents refer to?

[daobrien]

https://tldp.org/LDP/abs/html/here-docs.html

Simple example:

cat << EOF
This is line 1
This is line 2
This is line 3
EOF

This writes 3 lines to STDOUT.
If you want (and more commonly), include EOF > text-file as part of the first line. That way the three lines are written to text-file instead.
HERE docs have a zillion uses.

[julian-cable]

Some of these references were already updated. I added some minor updates.