Keyword descriptions of let-bang and friends should be consistent and not specifically refer to asyncs #5343

As discussed in #4427 (review)

Some computation-expression keyword descriptions are inconsistent with each other in how they are described, and this should be fixed. For example:

• let! says it's "Used in asynchronous workflows ..."
• use! says it's "Used ... in asynchronous workflows and computation expressions ..."
• return! says it's "Used to indicate a computation expression ..."
• yield! says it's "Used in a computation expression ..."
• and finally, match! says it's "Used in computation expressions ..."

All of these descriptions should be unified to either all say "Used in computation expressions", or "Used in a computation expression" (doesn't really matter which; just as long as they're consistent).