docs: document breaking change for nvim_create_autocmd callback (#27484)

https://github.com/neovim/neovim/pull/27428 changed the semantics of
callbacks passed to nvim_create_autocmd such that any truthy value will
delete the autocommand (rather than just the literal boolean value
`true`). Update the documentation accordingly and add an entry to
`news.txt`.

The behavior is now consistent between nvim_create_autocmd and
nvim_buf_attach.

runtime/doc/api.txt

@@ -2099,8 +2099,9 @@ nvim_buf_attach({buffer}, {send_buffer}, {*opts})          *nvim_buf_attach()*
                        will be `nvim_buf_changedtick_event`. Not for Lua
                        callbacks.
       • {opts}         Optional parameters.
-                       • on_lines: Lua callback invoked on change. Return `true` to
-                         detach. Args:
+                       • on_lines: Lua callback invoked on change. Return a
+                         truthy value (not `false` or `nil`)
+                         to detach. Args:
                          • the string "lines"
                          • buffer handle
                          • b:changedtick
@@ -2113,7 +2114,8 @@ nvim_buf_attach({buffer}, {send_buffer}, {*opts})          *nvim_buf_attach()*
 
                        • on_bytes: Lua callback invoked on change. This
                          callback receives more granular information about the
-                         change compared to on_lines. Return `true` to
+                         change compared to on_lines. Return a truthy value
+                         (not `false` or `nil`) to
                          detach. Args:
                          • the string "bytes"
                          • buffer handle
@@ -3457,9 +3459,9 @@ nvim_create_autocmd({event}, {*opts})                  *nvim_create_autocmd()*
                    troubleshooting).
                  • callback (function|string) optional: Lua function (or
                    Vimscript function name, if string) called when the
-                   event(s) is triggered. Lua callback can return true to
-                   delete the autocommand, and receives a table argument with
-                   these keys:
+                   event(s) is triggered. Lua callback can return a truthy
+                   value (not `false` or `nil`) to delete the
+                   autocommand. Receives a table argument with these keys:
                    • id: (number) autocommand id
                    • event: (string) name of the triggered event
                      |autocmd-events|

runtime/doc/news.txt

@@ -122,6 +122,9 @@ The following changes may require adaptations in user config or plugins.
 
 • |shm-q| now fully hides macro recording message instead of only shortening it.
 
+• Returning any truthy value from a callback passed to |nvim_create_autocmd()|
+  (rather than just `true`) will delete the autocommand.
+
 ==============================================================================
 BREAKING CHANGES IN HEAD                                    *news-breaking-dev*