add default message when the summary of a protocol/convenience method is missing

[ArcturusZhang]

Description

The issue this PR is trying to resolve was first found in this PR: #4573

Previously when an operation in tsp has no @doc decorator, we will generate nothing in summary of xml doc.
This works fine in our existing cases, but in the new pattern for non-azure libraries, a convenience method could have no parameters because it no longer takes cancellationToken.
Our CodeWriter is designed in this way: when we told it to write a xml tag summary but it turns out to write nothing like:

/// <summary> </summary>

it will remove this line as if we never let it to write this.
This would be a trouble if a method takes no parameters:

• summary will not be written because it does not have one
• parameters will not be written because it does not have one
• exceptions will not be written because it does not throw exceptions (exceptions usually come from parameter validation)

And at the end, this public method is written without a xml doc, and this leads to a compilation error.

Therefore this PR introduces a default message for the operation, such as: The {name} method, so that we at least will write summary for a method.

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