Update the comments on F14 tables to be accurate.

Summary:
You can verify this behavior directly:
```
folly::F14FastSet<int> s;
for (int i = 0; i < 1000; ++i) {
  s.insert(i);
}
#define OUT(x) std::cout << #x << " -> " << (x)
OUT(s.bucket_count());
s.erase(s.begin(), s.end());
OUT(s.bucket_count());
s.clear();
OUT(s.bucket_count());
```
Output:
```
s.bucket_count() -> 1280
s.bucket_count() -> 1280
s.bucket_count() -> 0
```
Feel free to commandeer if you want to update the comment to suggest the erase idiom to avoid deallocating capacity.

Reviewed By: Gownta

Differential Revision: D52513904

fbshipit-source-id: 1cf61c7d4fade06b576b6751daac61020c4900a6

folly/container/F14Map.h

@@ -263,7 +263,7 @@ class F14BasicMap {
    * Remove all elements.
    * @methodset Modifiers
    *
-   * Does not free heap-allocated memory; capacity is unchanged.
+   * Frees heap-allocated memory; bucket_count is returned to 0.
    */
   void clear() noexcept { table_.clear(); }
 

folly/container/F14Set.h

@@ -258,7 +258,7 @@ class F14BasicSet {
   /**
    * Remove all elements.
    *
-   * Does not free heap-allocated memory; capacity is unchanged.
+   * Frees heap-allocated memory; bucket_count is returned to 0.
    *
    * @methodset Modifiers
    */