Improve docstring; don't bother caching

If we pass in different CWDs in the same git repo, we'll end up firing
off git multiple times. Let the consumer worry about it if it's that
important.

src/matrix_common/versionstring.py

@@ -12,7 +12,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import functools
 import logging
 import subprocess
 from typing import Optional
@@ -27,7 +26,6 @@
 logger = logging.getLogger(__name__)
 
 
-@functools.lru_cache()
 def get_distribution_version_string(
     distribution_name: str, cwd: Optional[str] = None
 ) -> str:
@@ -46,9 +44,11 @@ def get_distribution_version_string(
 
     Args:
         distribution_name: The name of the distribution package to check the version of
-        cwd: if provided, the directory to run all git commands in. If omitted, the
-             function will attempt to locate the distribution's source on disk and use
-             that location instead, but this is not reliable.
+
+        cwd: if provided, the directory to run all git commands in. `cwd` may also be
+            the path to a file, in which case `os.path.dirname(cwd)` is used instead.
+            If omitted, the function will attempt to locate the distribution's source
+            on disk and use that location instead---but this fallback is not reliable.
 
     Raises:
         importlib.metadata.PackageNotFoundError if the given distribution name doesn't