man: let's tone down the recommendation to use Type=exec a bit

man: let's tone down the recommendation to use Type=exec a bit

This is a follow-up for #28596.

I think the suggestion to use Type=exec uses too strong wording:
Type=exec has non-trivial drawbacks over Type=simple, and they deserve
to be mentioned.

Hence drop the <emphasis> and turn this around so that Type=exec is
*recommended*, but Type=simple is not expressly discouraged, because
there are plenty reasons to use it.

Add a brief discussion where Type=simple might be preferable.

Also, fix the outright unruth that Type=exec was the "simplest and
fastest", because it certainly is a lot, but not that.