[wg/social] Realign with charter template re security/privacy

[plehegar]

There was some ancient text there that needed to be relaligned.

Address the comments:

• I'm not sure a "substantive change" can (or should) "contain a section" on privacy and security; I think it's more likely that each substantive change with privacy and security implications would be reflected in a privacy & security considerations section
• The wording on security and privacy is very odd (only one security and one privacy section is needed, per spec, although these must be separate sections). Looks like a partial edit to me, and from an older charter template at that.

cc w3c/strategy#435

[tantek]

minor edit for better consistency with Web Platform Design Principles https://www.w3.org/TR/design-principles/#priority-of-constituencies