Clarify in the manual how map_values() behaves differently from map().

[jbrains]

Fixes #2466

[wader]

LGTM 👍

[itchyny]

      When any other type of value is passed to it, `map_values(x)`
      behaves the same as `map(x)`.

Not always. At least I can give a counterexample as follows.

 $ jq -n -c '[1,2,3] | map_values(.,.+10)'
[1,2,3]
 $ jq -n -c '[1,2,3] | map(.,.+10)' 
[1,11,2,12,3,13]

[pkoppstein]

Closing as this issue was specifically dealt with subsequently in 8345a8a

@jbrains - If the current version of manual.yml is still not to your liking, please feel free to submit a new PR. Thanks.