[Electron] Add number of bars of signal strength lookup table to RSSI docs

[technobly]

As defined in the AT command manual, we use this table to blink the number of bars of signal strength when the MODE button is pressed once https://docs.particle.io/reference/changelog/#check-signal-strength

Need to add that lookup table here: https://docs.particle.io/reference/firmware/electron/#rssi-

Any signal strength below -105dBm results in the modem terminating the connection with the tower.

/*   signal strength (u-Blox Sara U2 and G3 modules)
     *   0: < -105 dBm
     *   1: < -93 dBm
     *   2: < -81 dBm
     *   3: < -69 dBm
     *   4: < - 57 dBm
     *   5: >= -57 dBm
     */

[dmiddlecamp]

oops, I also just created this ticket, I'll close mine:

https://github.com/spark/docs/issues/638