Add a documentation article for @_spi

Also remove the `Create SwiftSyntax Release.md` because it describes how to vendor the parser library and we’re no longer doing that.

Author: ahoppen

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SwiftSyntaxDoccIndexTemplate.md

@@ -17,13 +17,17 @@ allows Swift tools to parse, inspect, generate, and transform Swift source code.
 ### Articles
 
 - <doc:Working-with-SwiftSyntax>
-- <doc:ChangingSwiftSyntax>
-- <doc:Create-SwiftSyntax-Release>
 
 ### Tutorials
 
 - <doc:Tutorial-Table-of-Contents>
 
+### Contributing
+
+These articles are intended for developers wishing to contribute to SwiftSyntax
+
+- <doc:ChangingSwiftSyntax>
+
 ### Syntax
 
 - <doc:SwiftSyntax/Syntax>

Sources/SwiftSyntax/Documentation.docc/Create SwiftSyntax Release.md

@@ -0,0 +1,9 @@
+# `@_spi` attribute
+
+swift-syntax extensively makes extensive use of the `@_spi` attribute. 
+
+Functions marked as `@_spi(RawSyntax)` (where `RawSyntax` can be any name) are considered *SPI* (System Programming Interface) and are only accessible if the module that declares them is imported as `@_spi(RawSyntax)`.
+
+Since functions marked as SPI are not part of the public API, swift-syntax makes no guarantee to their source stability. swift-syntax makes no effort to keep its SPI stable.
+
+Declarations are typically marked as SPI because they have some kind of caveat that makes them unsafe to use in general. For example, when accessing ``RawSyntax`` nodes, you need to manually guarantee that the ``SyntaxArena`` they live in doesn’t get de-allocated. Other declarations have an `@_spi` to share them between different modudules within the swift-syntax package but those APIs shouldn’t be accessed publicly.

Sources/SwiftSyntax/Documentation.docc/SPI.md

@@ -17,13 +17,17 @@ allows Swift tools to parse, inspect, generate, and transform Swift source code.
 ### Articles
 
 - <doc:Working-with-SwiftSyntax>
-- <doc:ChangingSwiftSyntax>
-- <doc:Create-SwiftSyntax-Release>
 
 ### Tutorials
 
 - <doc:Tutorial-Table-of-Contents>
 
+### Contributing
+
+These articles are intended for developers wishing to contribute to SwiftSyntax
+
+- <doc:ChangingSwiftSyntax>
+
 ### Syntax
 
 - <doc:SwiftSyntax/Syntax>