Grammatical fixes in documentation comments

[donatik27]

Changes Made

File: rayon-core/src/thread_pool/mod.rs

1. Changed compound term formatting:

   • Old: user-managed thread pools
   • New: user-managed thread-pools
     Reason: Maintaining consistent hyphenation of "thread-pool" throughout the documentation, as this compound term is used with hyphens elsewhere in the codebase.

2. Added hyphenation for compound adjective:

   • Old: user created
   • New: user-created
     Reason: The term "user-created" should be hyphenated to correctly form a compound adjective, improving readability and grammatical accuracy.

[cuviper]

Hi - the actual changes look fine, but the formatting looks mangled in your PR description, and it's excessive in the first place. Just the "Reason" statements would be sufficient.

[donatik27]

Hi - the actual changes look fine, but the formatting looks mangled in your PR description, and it's excessive in the first place. Just the "Reason" statements would be sufficient.

Done