Stabilise SSS in list of client features

[HarHarLinks]

matrix.org/CONTENT.md

Lines 164 to 173 in f758eca

	[extra.features]
	e2ee = false
	spaces = false
	voip_1to1 = false
	voip_jitsi = false
	threads = false
	sso = false
	oauth = false
	multi_account = false
	multi_language = false

MSC4186 is merged, and SSS is of high interest due to being part of "Matrix 2.0".

I'm not sure which implementations have stable support.

Related: #3230