Res: option in play

In the Sonic Pi Tutorial, A.5 Squelch that Bass in Acid Bass, the play option res: is listed, but it is not listed under play in the Help screen.
It seems to work, but what does this mean?

Quite a number of Sonic Pi’s built in synths employ a res or resonance opt. They are usually documented in the synth section of the help system See below for tb303

TB-303 Emulation

res: Filter resonance as a value between 0 and 1. Large amounts of resonance (a res: near 1) can create a whistling sound around the cutoff frequency. Smaller values produce less resonance.

Default: 0.9
Must be zero or greater, must be a value less than 1
May be changed whilst playing
Has slide options to shape changes

As far as what res does this may help:

What does resonance mean in synthesizer?

RESONANCE – Achieved by applying FEEDBACK to a FILTER: the effect of resonance “boosts” the frequencies around the CUTOFF frequency. As resonance increases, the feedback increases to a point that the filter can generate a sine-wave tone. This is called “self-oscillation”.

If you play around with the code below you can alter the res: opt between 0 and 0.9999999 say (must be < 1) and you can hear how as it aproaches 1 you gert a whistling as the system starts to go into oscillation… By default for :tb303 it is set to 0.9

use_synth :tb303
play 72,res: 0.95,cutoff: 100 #adjust between 0 and just less than 1

This live loop illustrates the res: value increasing then decreasing as a series of notes play. Look out for the whistle effect as the value gets over about 0.94

use_synth :tb303
live_loop :test do
  r=(ring 0,0.5,0.9,0.92,0.94,0.96,0.98,0.99).mirror
  play (scale :c2,:minor_pentatonic,num_octaves: 3).mirror.tick,res: r.look,cutoff: 100,release: 0.5
  sleep 0.5

Thank you very much Robin!
I found res: in the synth section of help and understood it.
I now understand that there may or may not be a res: depending on the tone.