Document `#![feature(bind_by_move_pattern_guards)]`

[Centril]

Stabilized in: rust-lang/rust#63118
RFC: rust-lang/rfcs#107
Tracking issue: rust-lang/rust#15287

[Centril]

This could probably be documented informally by adding a sentence or two in the sentence in the section on match in the paragraph on if guards:

Match arms can accept match guards to further refine the criteria for matching a case. Pattern guards appear after the pattern and consist of a bool-typed expression following the if keyword. A pattern guard may refer to the variables bound within the pattern they follow.