Fix PauliEvolutionGate (using product formulas) for all-identity Pauli terms (backport #13634)

[mergify]

Summary

The PauliEvolutionGate, if used with a product formula synthesis (this is the default),
did not correctly handle all-identity terms in the operator:

• it was off by a factor 2, and
• it broke for parameterized times (fixes Failed preparing ansatz circuit using PauliEvolutionGate when 'time' is taken as a Parameter('t'). #13625)

Details and comments

Some minor corrections to docs are included.

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This is an automatic backport of pull request #13634 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 12765766650

Details

• 10 of 12 (83.33%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 88.909%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/circuit/src/operations.rs	3	5	60.0%

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	92.48%

Totals
Change from base Build 12765466656:	-0.001%
Covered Lines:	79155
Relevant Lines:	89029

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💛 - Coveralls