Increase documentation of ParameterVector and ParameterVectorElement (backport #13761)

[mergify]

Summary

These were somewhat lacking before, despite being fairly heavily used. ParameterVectorElement was not publicly documented, despite adding additional properties that were intended for public use.

Details and comments

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This is an automatic backport of pull request #13761 done by [Mergify](https://mergify.com).

[qiskit-bot]

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[coveralls]

Pull Request Test Coverage Report for Build 13072048612

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 88.934%

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Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	93.21%
crates/qasm2/src/lex.rs	5	92.23%

Totals
Change from base Build 13013025829:	0.01%
Covered Lines:	79172
Relevant Lines:	89023

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💛 - Coveralls