Add information about limitations for identifiers to SdfPath's Path Syntax Section

[instinct-vfx]

Description of Issue

As discussed with @spiffmon on Slack it would be good to have information about limitations on identifiers (as outlined here: TfIsValidIdentifier() documentation ) to SdfPath's Path Syntax description.

(Also as a memo for the backs of everyone's head it would be good to have in a Usd-Basics document that does not really exist yet.)

[nvmkuruc]

In case it's helpful--
https://openusd.org/release/api/_usd__page__u_t_f_8.html#Usd_UTF_8_Identifiers
https://openusd.org/release/api/_usd__page__u_t_f_8.html#Usd_UTF_8_Encoding_Reference

[jesschimein]

Filed as internal issue #USD-10226