Small issues with mb_convert_* documentation

[alecpl]

1. mb_convert_encoding()

to_encoding and from_encoding mentions "type of coding" and "character code" while it should be just "encoding" or "character encoding". It might be confusing.

2. mb_convert_variables()

from_encoding mentions non-existing from-coding label. Also, the argument description could be written better. Last, but not least, in "When from_encoding is omitted, detect_order is used.", how it can be omitted if the argument type is array|string? I think it could be clarified.