Fix adhoc output_... example, document run_shell_command workdir chan…

…ges (pantsbuild#18894)

This does two minor doc tweaks related to the shell/adhoc tool work in
2.16:

- the `adhoc_tool` docs had flipped the file vs directory for the
`output_...` example (fixes pantsbuild#18890)
- the `run_shell_command` workdir field (including its default
behaviour) has changed in 2.16 (pantsbuild#18840, pantsbuild#18850), and this is added to
the changelog, rather than migrated (quite hard pantsbuild#18860) or properly
deprecated (also a bit awkward, and this is an `experimental_...`
target)

src/python/pants/backend/adhoc/target_types.py

@@ -257,8 +257,8 @@ class AdhocToolTarget(Target):
                 {AdhocToolRunnableField.alias}=":python_source",
                 {AdhocToolArgumentsField.alias}=[""],
                 {AdhocToolExecutionDependenciesField.alias}=[":scripts"],
-                {AdhocToolOutputDirectoriesField.alias}=["logs/my-script.log"],
-                {AdhocToolOutputFilesField.alias}=["results/"],
+                {AdhocToolOutputDirectoriesField.alias}=["results/"],
+                {AdhocToolOutputFilesField.alias}=["logs/my-script.log"],
             )
 
             shell_sources(name="scripts")

src/python/pants/notes/2.16.x.md

@@ -89,6 +89,8 @@ tool may be invoked as a "runnable" by the `adhoc_tool` target type, while also
 - Defining arbitrary shell commands as tests which can be invoked using `pants test` via the
 new `experimental_test_shell_command` target type.
 
+- The `workdir` field to `run_shell_command` has been switched to match the documented behavior: it now runs from the same directory as the `BUILD` file by default. To preserve the old behavior, explicitly set `workdir="/"`. The stabilisation also fixes other bugs with relative paths passed to the `workdir` field and thus may require updates.
+
 Ad-hoc tool support, with the `adhoc_tool` or `shell_command` targets, can be used to compile third party
 libraries in unsupported languages, run tools from ecosystems Pants doesn't yet directly support, and other similar code-generation tasks. 
 These targets can be used for first-party code generation, making use of existing backends that define