Document the syntax of OpenApiError.Pointer

[KalleOlaviNiemitalo]

The OpenApiError.Pointer property currently has a minimal XML documentation comment:

OpenAPI.NET/src/Microsoft.OpenApi/Models/OpenApiError.cs

Lines 34 to 37 in be6a700

	/// <summary>
	/// Pointer to the location of the error.
	/// </summary>
	public string Pointer { get; set; }

I would like to see documentation of the possible syntaxes here. I have an MSBuild task that tries to parse this string and pass line and column numbers to TaskLoggingHelper.LogError.

Currently, the possibilities seem to be:

• #line= + line number counting from 0. This is defined in RFC 5147, although that does not formally apply to application/json.
• #char= + line number counting from 0. Will be fixed in OpenApiTextReaderReader inconsistently sets "#char=" vs. "#line=" in OpenApiError.Pointer #495.
• # + JSON Pointer (RFC 6901), except the ~ and / characters are not escaped as ~0 and ~1.

(Some errors have more detailed location information in OpenApiError.Message. Filed #502 for making that easier to access.)

[KalleOlaviNiemitalo]

Verified that OpenApiError.Pointer in Microsoft.OpenApi.Readers 1.2.2 does not escape / as ~1 if the error occurs within a Path Item Object, whose key in the Paths Object always starts with a slash.