[Docs] Explicitly document libclang ABI and API stability (#141657)

Our current docs leave a lot of latitude ("relatively stable") without
explaining what the goals are for this stability. This patch adds some
basic documentation explaining that there are some changes which can
impact ABI and API stability that we reserve the right to make, lists
some scenarios we explicitly do not support, but otherwise tries to
assure the reader that the APIs and ABI are stable.

Author: AaronBallman

clang/docs/LibClang.rst

@@ -4,7 +4,7 @@
 Libclang tutorial
 =================
 The C Interface to Clang provides a relatively small API that exposes facilities for parsing source code into an abstract syntax tree (AST), loading already-parsed ASTs, traversing the AST, associating physical source locations with elements within the AST, and other facilities that support Clang-based development tools.
-This C interface to Clang will never provide all of the information representation stored in Clang's C++ AST, nor should it: the intent is to maintain an API that is relatively stable from one release to the next, providing only the basic functionality needed to support development tools.
+This C interface to Clang will never provide all of the information representation stored in Clang's C++ AST, nor should it: the intent is to maintain an API that is :ref:`relatively stable <Stability>` from one release to the next, providing only the basic functionality needed to support development tools.
 The entire C interface of libclang is available in the file `Index.h`_
 
 Essential types overview
@@ -358,3 +358,49 @@ Complete example code
 
 
 .. _Index.h: https://github.com/llvm/llvm-project/blob/main/clang/include/clang-c/Index.h
+
+.. _Stability:
+
+ABI and API Stability
+---------------------
+
+The C interfaces in libclang are intended to be relatively stable. This allows
+a programmer to use libclang without having to worry as much about Clang
+upgrades breaking existing code. However, the library is not unchanging. For
+example, the library will gain new interfaces over time as needs arise,
+existing APIs may be deprecated for eventual removal, etc. Also, the underlying
+implementation of the facilities by Clang may change behavior as bugs are
+fixed, features get implemented, etc.
+
+The library should be ABI and API stable over time, but ABI- and API-breaking
+changes can happen in the following (non-exhaustive) situations:
+
+* Adding new enumerator to an enumeration (can be ABI-breaking in C++).
+* Removing an explicitly deprecated API after a suitably long deprecation
+  period.
+* Using implementation details, such as names or comments that say something
+  is "private", "reserved", "internal", etc.
+* Bug fixes and changes to Clang's internal implementation happen routinely and
+  will change the behavior of callers.
+* Rarely, bug fixes to libclang itself.
+
+The library has version macros (``CINDEX_VERSION_MAJOR``,
+``CINDEX_VERSION_MINOR``, and ``CINDEX_VERSION``) which can be used to test for
+specific library versions at compile time. The ``CINDEX_VERSION_MAJOR`` macro
+is only incremented if there are major source- or ABI-breaking changes. Except
+for removing an explicitly deprecated API, the changes listed above are not
+considered major source- or ABI-breaking changes. Historically, the value this
+macro expands to has not changed, but may be incremented in the future should
+the need arise. The ``CINDEX_VERSION_MINOR`` macro is incremented as new APIs
+are added. The ``CINDEX_VERSION`` macro expands to a value based on the major
+and minor version macros.
+
+In an effort to allow the library to be modified as new needs arise, the
+following situations are explicitly unsupported:
+
+* Loading different library versions into the same executable and passing
+  objects between the libraries; despite general ABI stability, different
+  versions of the library may use different implementation details that are not
+  compatible across library versions.
+* For the same reason as above, serializing objects from one version of the
+  library and deserializing with a different version is also not supported.