Fix u16 docs and improve docs formatting

[christophebedard]

1. I noticed that the docs for the size param for the u16 string sequence functions in rosidl_runtime_c are wrong. The size param refers to the size of the sequence, not the string. This is clear when looking at the implementations and at the docs of the other similar sequence functions.
2. I also modified the description of the n param for rosidl_runtime_c__U16String__assignn_from_char to clarify that it is the number of 8-bit chars in the string. I also mentioned that the function will return false if n is not even.
3. Finally, I also added some missing * after /* in docs (to get the usual /**).

[fujitatomoya]

lgtm

[clalancette]

Looks good to me.

Since this is only a documentation change, and the Rpr jobs are working again, I'm going to merge it in with just that passing. Thanks for updating this!