Man-pages: make consequences of eval $(opam env --switch=SWITCH --set-switch) clearer

[pitag-ha]

Hi. I've been using eval $(opam env --switch=SWITCH --set-switch) for a while to change the switch since that is what it says in the man-pages. Is that still recommended, also when using local switches? Might it be a good idea to point out in the man-pages that it overrides the OPAMSWITCH variable? I've talked about that with @kit-ty-kate.

[rjbou]

Manpage updated!

[rjbou]

thanks!