Add note on enumerable collections

[BillWagner]

Fixes #40541

Add an include note for all samples in this section. Point out that all use IEnumerable based collections, so some restrictions will apply when an IQueryable source is used, and expressions are created.

Also, point out that each IQueryProvider has unique restrictions.

Finally, update the left join method syntax sample to use SelectMany which is closer to the lowered code.

Fixes #41095

Move paragraph about statement lambdas to the limitations section.

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docs/csharp/advanced-topics/expression-trees/index.md	Expression Trees
docs/csharp/linq/standard-query-operators/converting-data-types.md	"Converting Data Types"
docs/csharp/linq/standard-query-operators/filtering-data.md	Filtering Data in C# with LINQ
docs/csharp/linq/standard-query-operators/grouping-data.md	docs/csharp/linq/standard-query-operators/grouping-data
docs/csharp/linq/standard-query-operators/index.md	Standard Query Operators Overview
docs/csharp/linq/standard-query-operators/join-operations.md	Join Operations in LINQ
docs/csharp/linq/standard-query-operators/partitioning-data.md	"Partitioning data"
docs/csharp/linq/standard-query-operators/projection-operations.md	Projection operations (C#)
docs/csharp/linq/standard-query-operators/quantifier-operations.md	Quantifier operations in LINQ (C#)
docs/csharp/linq/standard-query-operators/set-operations.md	docs/csharp/linq/standard-query-operators/set-operations
docs/csharp/linq/standard-query-operators/sorting-data.md	Sorting Data (C#)

[IEvangelist]

LGTM, just need to use an instead of a as it should read "an IEnumerable".