Added documentation for System.Text.Rune struct

[jozkee]

Documented APIs:

• op_Equality(System.Text.Rune,System.Text.Rune)
• op_Explicit(System.UInt32)~System.Text.Rune
• op_Explicit(System.Char)~System.Text.Rune
• op_Explicit(System.Int32)~System.Text.Rune
• op_GreaterThan(System.Text.Rune,System.Text.Rune)
• op_GreaterThanOrEqual(System.Text.Rune,System.Text.Rune)
• op_Inequality(System.Text.Rune,System.Text.Rune)
• op_LessThan(System.Text.Rune,System.Text.Rune)
• op_LessThanOrEqual(System.Text.Rune,System.Text.Rune)
• ToLower(System.Text.Rune,System.Globalization.CultureInfo)
• ToLowerInvariant(System.Text.Rune)
• ToUpper(System.Text.Rune,System.Globalization.CultureInfo)
• ToUpperInvariant(System.Text.Rune)

[jozkee]

@carlossanlop @rpetrusha @mairaw

[jozkee]

@GrabYourPitchforks note that I am referring to Rune as a 32-bit Unicode character. Please correct me if I am wrong or if we should omit that.

[carlossanlop]

I left some comments, @jozkee.

[rpetrusha]

I thought that the explicit conversion description with "Defines an explicit conversion..." is good, @jozkee. I also noted where exceptions should be documented.

[jozkee]

Adding exceptions for op_Explicit

[jozkee]

@rpetrusha I've addressed the feedback. Please tell me if something else is missing. Also plese verify the wording in the exceptions added to op_Explicit.

[carlossanlop]

LGTM.

[rpetrusha]

This looks good, @jozkee. Thanks for making the additional changes. I'll merge now.