Port System.Linq 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: #105984 (comment)

Outdated

Scroll right to see the details:

API	Summary	Parameters	TypeParameters	ReturnValue
M:System.Linq.Enumerable.AggregateBy3(System.Collections.Generic.IEnumerable{0},System.Func{0,1},2,System.Func{2,0,2},System.Collections.Generic.IEqualityComparer{1})	Missing	Missing	Missing	Missing
M:System.Linq.Enumerable.AggregateBy3(System.Collections.Generic.IEnumerable{0},System.Func{0,1},System.Func{1,2},System.Func{2,0,2},System.Collections.Generic.IEqualityComparer{1})	Missing	Missing	Missing	Missing
M:System.Linq.Enumerable.CountBy2(System.Collections.Generic.IEnumerable{0},System.Func{0,1},System.Collections.Generic.IEqualityComparer{1})	Missing	Missing	Missing	Missing
M:System.Linq.Enumerable.Index1(System.Collections.Generic.IEnumerable{0})	Missing	Missing	Missing	Missing
M:System.Linq.Queryable.AggregateBy3(System.Linq.IQueryable{0},System.Linq.Expressions.Expression{System.Func{0,1}},System.Linq.Expressions.Expression{System.Func{1,2}},System.Linq.Expressions.Expression{System.Func{2,0,2}},System.Collections.Generic.IEqualityComparer{1})	Missing	Missing	Missing	Missing
M:System.Linq.Queryable.AggregateBy3(System.Linq.IQueryable{0},System.Linq.Expressions.Expression{System.Func{0,1}},2,System.Linq.Expressions.Expression{System.Func{2,0,2}},System.Collections.Generic.IEqualityComparer{1})	Missing	Missing	Missing	Missing
M:System.Linq.Queryable.CountBy2(System.Linq.IQueryable{0},System.Linq.Expressions.Expression{System.Func{0,1}},System.Collections.Generic.IEqualityComparer{1})	Missing	Missing	Missing	Missing
M:System.Linq.Queryable.Index1(System.Linq.IQueryable{0})	Missing	Missing	Missing	Missing

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[carlossanlop]

Some APIs from the table might get addressed with dotnet/dotnet-api-docs#10216

[carlossanlop]

P7 Undoc APIs as of 08/20:

Summary	Parameters	TypeParameters	ReturnValue	API
Missing	Missing	Missing	Missing	M:System.Linq.Enumerable.AggregateBy3(System.Collections.Generic.IEnumerable{0},System.Func{0,1},System.Func{1,2},System.Func{2,0,2},System.Collections.Generic.IEqualityComparer{1})
Missing	Missing	Missing	Missing	M:System.Linq.Enumerable.AggregateBy3(System.Collections.Generic.IEnumerable{0},System.Func{0,1},2,System.Func{2,0,2},System.Collections.Generic.IEqualityComparer{1})
Missing	Missing	Missing	Missing	M:System.Linq.Enumerable.CountBy2(System.Collections.Generic.IEnumerable{0},System.Func{0,1},System.Collections.Generic.IEqualityComparer{1})
Present	Present	Present	Missing	M:System.Linq.Enumerable.Index1(System.Collections.Generic.IEnumerable{0})
Missing	Missing	Missing	Missing	M:System.Linq.Queryable.AggregateBy3(System.Linq.IQueryable{0},System.Linq.Expressions.Expression{System.Func{0,1}},2,System.Linq.Expressions.Expression{System.Func{2,0,2}},System.Collections.Generic.IEqualityComparer{1})
Missing	Missing	Missing	Missing	M:System.Linq.Queryable.AggregateBy3(System.Linq.IQueryable{0},System.Linq.Expressions.Expression{System.Func{0,1}},System.Linq.Expressions.Expression{System.Func{1,2}},System.Linq.Expressions.Expression{System.Func{2,0,2}},System.Collections.Generic.IEqualityComparer{1})
Present	Present	Present	Missing	M:System.Linq.Queryable.Index1(System.Linq.IQueryable{0})