C++: Do not use [=] or [=,this] to capture 'this'

[pdillinger]

Summary: C++20 deprecates [=] capturing 'this' (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0806r2.html) but the recommended alternative [=,this] is not standards-legal prior to C++20 (though it is widely accepted with warning).

For portability (and readability), style should forbid using [=] or [=,this] to capture 'this'. Any by-value captures must be explicitly listed if also capturing 'this', though 'this' can still be implicitly default-captured by reference, as in [&, some_var_to_copy].

Test Plan: documentation only

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