Guarantee try_reserve preserves the contents on error

Update doc comments to make the guarantee explicit. However, some
implementations does not have the statement though.

* `HashMap`, `HashSet`: require guarantees on hashbrown side.
* `PathBuf`: simply redirecting to `OsString`.

Fixes #99606.

library/alloc/src/collections/binary_heap.rs

@@ -1010,7 +1010,8 @@ pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveE
     /// current length. The allocator may reserve more space to speculatively
     /// avoid frequent allocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///

library/alloc/src/collections/vec_deque/mod.rs

@@ -794,7 +794,8 @@ pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveE
     /// in the given deque. The collection may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///

library/alloc/src/string.rs

@@ -1080,7 +1080,8 @@ pub fn reserve_exact(&mut self, additional: usize) {
     /// current length. The allocator may reserve more space to speculatively
     /// avoid frequent allocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///

library/alloc/src/vec/mod.rs

@@ -875,7 +875,8 @@ pub fn reserve_exact(&mut self, additional: usize) {
     /// in the given `Vec<T>`. The collection may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns
-    /// `Ok(())`. Does nothing if capacity is already sufficient.
+    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
+    /// preserves the contents even if an error occurs.
     ///
     /// # Errors
     ///

library/std/src/ffi/os_str.rs

@@ -290,7 +290,8 @@ pub fn reserve(&mut self, additional: usize) {
     /// in the given `OsString`. The string may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns `Ok(())`.
-    /// Does nothing if capacity is already sufficient.
+    /// Does nothing if capacity is already sufficient. This method preserves
+    /// the contents even if an error occurs.
     ///
     /// See the main `OsString` documentation information about encoding and capacity units.
     ///

library/std/src/sys_common/wtf8.rs

@@ -236,7 +236,8 @@ pub fn reserve(&mut self, additional: usize) {
     /// in the given `Wtf8Buf`. The `Wtf8Buf` may reserve more space to avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional`. Does nothing if
-    /// capacity is already sufficient.
+    /// capacity is already sufficient. This method preserves the contents even
+    /// if an error occurs.
     ///
     /// # Errors
     ///