tidy-up: whitespace, fix CI spacecheck for docs

Also: fixup CI spacecheck to apply to docs again.

Closes #15423

MQTT: remove trailing newline

mqtt: fix mqtt.md wording and add clearer explanation

Some of the wording in the mqtt.md confused me as to how the commands
were used and what they did, so I cleared up some of the wordi

mqtt: fix mqtt.md wording and add clearer explanation

Some of the wording in the mqtt.md confused me as to how the commands
were used and what they did, so I cleared up some of the wording to
better explain what each command does.

Closes #15451

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docs/internals: new subdirectory

For libcurl internal APIs and concepts.

Closes #14552