[mojo-stdlib][docs] Minor doc edits for packages (#35577)

Some minor doc edits I noticed when comparing the staged site of docs
vs. last release as some things have moved around. While here, update the
changelog for `parallel_memcpy` being *moved*, not *removed* from the standard
library.

modular-orig-commit: f3a53c28b5db3b7477a0675b2df4bd632cde5eb3

docs/changelog.md

@@ -147,8 +147,8 @@ modular update mojo
     fn var_kwparams[**kwparams: Int](): ...
     ```
 
-  - Added new `collections.OptionalReg` type, a register-passable alternative
-    to `Optional`.
+- Added new `collections.OptionalReg` type, a register-passable alternative
+  to `Optional`.
 
 ### 🦋 Changed
 
@@ -231,9 +231,11 @@ modular update mojo
   - `rand` and `randn` functions in the `random` package that return a `Tensor`
      have moved to the `tensor` package.
   - `Buffer`, `NDBuffer`, and friends have moved from the `memory` package
-    into a new `buffer` package.
+     into a new `buffer` package.
   - The `trap` function has been renamed to `abort`.  It also has moved from the
     `debug` module to the `os` module.
+  - `parallel_memcpy` has moved from the `memory` package into
+     the `buffer` package.
 
 ### ❌ Removed
 
@@ -255,10 +257,8 @@ modular update mojo
 - `memcpy` on `Buffer` has been removed in favor of just overloads for `Pointer`
   and `DTypePointer`.
 
-- `parallel_memcpy` has been removed from the Standard Library for now.
-
-  - The functions `max_or_inf`, `min_or_neginf` have been removed from
-    `math.limit` and just inlined into their use in SIMD.
+- The functions `max_or_inf`, `min_or_neginf` have been removed from
+  `math.limit` and just inlined into their use in SIMD.
 
 ### 🛠️ Fixed
 

stdlib/src/testing/__init__.mojo

@@ -11,6 +11,8 @@
 # limitations under the License.
 # ===----------------------------------------------------------------------=== #
 
+"""Implements the testing package."""
+
 from .testing import (
     assert_true,
     assert_false,

stdlib/src/utils/small_string.mojo

@@ -11,6 +11,10 @@
 # limitations under the License.
 # ===----------------------------------------------------------------------=== #
 
+"""Implements a string that has a small-string optimization which
+   avoids heap allocations for short strings.
+"""
+
 from sys.info import sizeof
 
 from memory import memcpy

stdlib/src/utils/stringref.mojo

@@ -11,8 +11,6 @@
 # limitations under the License.
 # ===----------------------------------------------------------------------=== #
 """Implements the StringRef class.
-
-These are Mojo built-ins, so you don't need to import them.
 """