Port System.Threading.Tasks docs for .NET 9 APIs

[carlossanlop]

Below is the list of APIs that still show up as undocumented in dotnet-api-docs and were introduced in .NET 9.

Full porting instructions can be found in the main issue.

This task needs to be finished the day before the RC2 snap (September 15th).

P7 Undoc APIs as of 08/20: #105985 (comment)

Outdated

Scroll right to see the details:

API	Summary	Parameters	TypeParameters	ReturnValue
M:System.Threading.Tasks.Task.WhenAll1(System.ReadOnlySpan{System.Threading.Tasks.Task{0}})	Present	Present	Missing	Present
M:System.Threading.Tasks.Task.WhenAny1(System.ReadOnlySpan{System.Threading.Tasks.Task{0}})	Present	Present	Missing	Present
M:System.Threading.Tasks.Task.WhenEach1(System.ReadOnlySpan{System.Threading.Tasks.Task{0}})	Present	Present	Missing	Present
M:System.Threading.Tasks.Task.WhenEach1(System.Threading.Tasks.Task{0}[])	Present	Present	Missing	Present
M:System.Threading.Tasks.Task.WhenEach1(System.Collections.Generic.IEnumerable{System.Threading.Tasks.Task{0}})	Present	Present	Missing	Present

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[carlossanlop]

P7 Undoc APIs as of 08/20:

Summary	Parameters	TypeParameters	ReturnValue	API
Present	Present	Missing	Present	M:System.Threading.Tasks.Task.WhenAll1(System.ReadOnlySpan{System.Threading.Tasks.Task{0}})
Present	Present	Missing	Present	M:System.Threading.Tasks.Task.WhenAny1(System.ReadOnlySpan{System.Threading.Tasks.Task{0}})
Present	Present	Missing	Present	M:System.Threading.Tasks.Task.WhenEach1(System.Threading.Tasks.Task{0}[])
Present	Present	Missing	Present	M:System.Threading.Tasks.Task.WhenEach1(System.ReadOnlySpan{System.Threading.Tasks.Task{0}})
Present	Present	Missing	Present	M:System.Threading.Tasks.Task.WhenEach1(System.Collections.Generic.IEnumerable{System.Threading.Tasks.Task{0}})

@dotnet/area-system-threading-tasks PTAL

[ericstj]

@jozkee some of these were params span overloads. 1c824d9
@stephentoub added WhenEach ab2bd48

All seem to have docs in the source - so we just need to port to the docs repo.

[ericstj]

Ah, I see - missing docs for the type parameter on all of these.