[clang-tidy][NFC] Sync ContainerSizeEmptyCheck with container-size-empty doc (llvm#118459)

Brought the class documentation in sync with the user documentation at
container-size-empty.rst:


https://github.com/llvm/llvm-project/blob/bfb26202e05ee2932b4368b5fca607df01e8247f/clang-tools-extra/docs/clang-tidy/checks/readability/container-size-empty.rst#L7-L14

Author: N-Dekker

clang-tools-extra/clang-tidy/readability/ContainerSizeEmptyCheck.h

@@ -18,10 +18,11 @@ namespace clang::tidy::readability {
 /// a call to `empty()`.
 ///
 /// The emptiness of a container should be checked using the `empty()` method
-/// instead of the `size()` method. It shows clearer intent to use `empty()`.
-/// Furthermore some containers may implement the `empty()` method but not
-/// implement the `size()` method. Using `empty()` whenever possible makes it
-/// easier to switch to another container in the future.
+/// instead of the `size()`/`length()` method. It shows clearer intent to use
+/// `empty()`. Furthermore some containers may implement the `empty()` method
+/// but not implement the `size()` or `length()` method. Using `empty()`
+/// whenever possible makes it easier to switch to another container in the
+/// future.
 class ContainerSizeEmptyCheck : public ClangTidyCheck {
 public:
   ContainerSizeEmptyCheck(StringRef Name, ClangTidyContext *Context);