Specify IGate meaning in the docs (backport #15462)

[mergify]

Summary

Closes #15449. While coming up with instructions on what this should be we came up with the full sentence, so it'll be easier to just directly implement the change.

Details and comments

This phrasing should leave it open that the exact interpretation is up to the vendor, but typically it refers to a single-qubit wait cycle (what time that is depends on the vendor and should be queried using the Target). This is also close to the previous docs, though we no longer state that IGates are not removed -- because they are removed in transpilation.

---

This is an automatic backport of pull request #15462 done by [Mergify](https://mergify.com).

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the following people are relevant to this code:

• @Cryoris
• @Qiskit/terra-core
• @ajavadia

[coveralls]

Pull Request Test Coverage Report for Build 20338185742

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.01%) to 88.307%

---

Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/parameter_expression.rs	1	82.3%
crates/transpiler/src/passes/unitary_synthesis.rs	1	93.32%
crates/qasm2/src/lex.rs	2	92.03%

Totals
Change from base Build 20242317114:	0.01%
Covered Lines:	96641
Relevant Lines:	109438

---

💛 - Coveralls