[llvm] [Docs] Clarify that `reassoc` isn't just for reassociation (PR #133168)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-devrel-x86-64` running on `ml-opt-devrel-x86-64-b1` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/175/builds/16169

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: Transforms/Vectorize/./VectorizeTests/18/27' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/b/ml-opt-devrel-x86-64-b1/build/unittests/Transforms/Vectorize/./VectorizeTests-LLVM-Unit-48532-18-27.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=27 GTEST_SHARD_INDEX=18 /b/ml-opt-devrel-x86-64-b1/build/unittests/Transforms/Vectorize/./VectorizeTests
--

Note: This is test shard 19 of 27.
[==========] Running 2 tests from 2 test suites.
[----------] Global test environment set-up.
[----------] 1 test from VPRecipeTest
[ RUN      ] VPRecipeTest.CastVPInterleaveRecipeToVPUser
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  VectorizeTests  0x00005651c2dcae08
1  VectorizeTests  0x00005651c2dc8705
2  libpthread.so.0 0x00007f62420ff140
3  VectorizeTests  0x00005651c29bdee4
4  VectorizeTests  0x00005651c2dde32f
5  VectorizeTests  0x00005651c2de6082
6  VectorizeTests  0x00005651c2de6667
7  VectorizeTests  0x00005651c2df3666
8  VectorizeTests  0x00005651c2ddd89c
9  VectorizeTests  0x00005651c2991dd6
10 libc.so.6       0x00007f6241bebd7a __libc_start_main + 234
11 VectorizeTests  0x00005651c299755a

--
exit: -11
--
shard JSON output does not exist: /b/ml-opt-devrel-x86-64-b1/build/unittests/Transforms/Vectorize/./VectorizeTests-LLVM-Unit-48532-18-27.json
********************


```

</details>

https://github.com/llvm/llvm-project/pull/133168