Clarify the post-condition of reserve() in Abseil hash containers.

The comments for `reserve()` in `flat_hash_map`, `flat_hash_set`, `node_hash_map`, and `node_hash_set` are updated to explicitly state that after calling `reserve(count)`, at least `count - size()` additional elements can be inserted without triggering a rehash.

PiperOrigin-RevId: 818932349
Change-Id: I76b014eace2367ca7aac8f5ba5cf6e5d9cbc50f1

Author: Abseil Team

absl/container/flat_hash_map.h

@@ -462,9 +462,7 @@ class ABSL_ATTRIBUTE_OWNER flat_hash_map
   //
   // Sets the number of slots in the `flat_hash_map` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed. After this
-  // returns, it is guaranteed that `count - size()` elements can be inserted
-  // into the `flat_hash_map` without another rehash.
+  // maximum load factor, and may rehash the container if needed.
   using Base::reserve;
 
   // flat_hash_map::at()

absl/container/flat_hash_set.h

@@ -396,9 +396,7 @@ class ABSL_ATTRIBUTE_OWNER flat_hash_set
   //
   // Sets the number of slots in the `flat_hash_set` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed. After this
-  // returns, it is guaranteed that `count - size()` elements can be inserted
-  // into the `flat_hash_set` without another rehash.
+  // maximum load factor, and may rehash the container if needed.
   using Base::reserve;
 
   // flat_hash_set::contains()

absl/container/node_hash_map.h

@@ -455,9 +455,7 @@ class ABSL_ATTRIBUTE_OWNER node_hash_map
   //
   // Sets the number of slots in the `node_hash_map` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed. After this
-  // returns, it is guaranteed that `count - size()` elements can be inserted
-  // into the `node_hash_map` without another rehash.
+  // maximum load factor, and may rehash the container if needed.
   using Base::reserve;
 
   // node_hash_map::at()

absl/container/node_hash_set.h

@@ -390,9 +390,7 @@ class ABSL_ATTRIBUTE_OWNER node_hash_set
   //
   // Sets the number of slots in the `node_hash_set` to the number needed to
   // accommodate at least `count` total elements without exceeding the current
-  // maximum load factor, and may rehash the container if needed. After this
-  // returns, it is guaranteed that `count - size()` elements can be inserted
-  // into the `node_hash_set` without another rehash.
+  // maximum load factor, and may rehash the container if needed.
   using Base::reserve;
 
   // node_hash_set::contains()