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Add documentation on OpenQASM 2 phase conventions (#10801) (#10913)

* Add documentation on OpenQASM 2 phase conventions

OpenQASM 2 as a language does not provide a way to specify global
phases, either of operations or the entire program.  The prose of the
defining paper talks about the `u1` and `rz` instructions from
`qelib1.inc` as having different global phases, however since the
language cannot represent this, the header file defines `rz` apparently
as a direct alias to `u1`.

Qiskit's position has always been that we interpret `rz` as using the
phase convention of `RZGate`, and `u1` uses the different convention of
`PhaseGate`.  This commit adds documentation to the pages on OpenQASM 2
about this difference, and includes an example of how to override
Qiskit's default if the generating code (e.g. pyZX) wants to use an
alternative phase convention.

* Comment on the number of overrides needed

Co-authored-by: Julien Gacon <gaconju@gmail.com>

---------

Co-authored-by: Julien Gacon <gaconju@gmail.com>
(cherry picked from commit 10461f18c)

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

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