Misc documentation fixes (no code change)

We use macros to encapsulate the appropriate type and size
information for the different CTL requests, but the macros
still need to be used with the _ctl() function call and an
encoder or decoder instance structure. As such, just listing
the macro defines is confusing. Adding some examples outside
the OpusEncoder overview page should help with this.

Also document that OPUS_SET_APPLICATION can take
OPUS_APPLICATION_RESTRICTED_LOWDELAY.