DOC: Improve the message printed when running `make` w/o `TARGET`

DOC: Improve the message printed when running `make` w/o `TARGET`

Rephrase the message to no longer talk about something that "is no longer
supported", but about what actually *is* supported.

Adjustments include:

- Removal of rare targets to make it easier to find the proper one.
- Reformatting to be easier to read (more newlines)
- Explanation of common non-default feature flags.

(cherry picked from commit 1ab76dd956042019aabbfc2a04dc6a3101ce1a02)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>