Further document ada-lang.c::value_assign_to_component & fix whitespaces.

Further document ada-lang.c::value_assign_to_component & fix whitespaces.

This patch improves the documentation of ada-lang.c's
value_assign_to_component to publish the fact that it also works
with not_lval values.

And touching this area of the code showed that there were a number
of whitespace issues, as well as a formatting issue of the main comment
(no leading '*' on each line). This patch fixes those while at it.

No functional change, however.

gdb/ChangeLog:

        * ada-lang.c (value_assign_to_component): Reformat and improve
        documentation. Remove all trailing spaces.