docs: Fix missing lines caused by undefined macros

docs: Fix missing lines caused by undefined macros

- Escape apostrophes at line start.

Some lines begin with a "'" (apostrophe, single quote), which is then
interpreted as a control character in *roff.

Such lines are interpreted as being a call to a macro, and if
undefined, the lines are removed from the output.

Bug: https://bugs.debian.org/926352
Signed-off-by: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>
Submitted-by: Alessandro Ghedini
Closes https://github.com/curl/curl/pull/4111