Update date references on parallel-rustc (#1348)

src/parallel-rustc.md

@@ -1,6 +1,6 @@
 # Parallel Compilation
 
-As of <!-- date: 2021-09 --> September 2021, The only stage of the compiler 
+As of <!-- date: 2022-05 --> May 2022, The only stage of the compiler
 that is already parallel is codegen. The nightly compiler implements query evaluation,
 but there is still a lot of work to be done. The lack of parallelism at other stages
 also represents an opportunity for improving compiler performance. One can try out the current
@@ -49,18 +49,18 @@ When a query `foo` is evaluated, the cache table for `foo` is locked.
 - If there *is* another query invocation for the same key in progress, we
   release the lock, and just block the thread until the other invocation has
   computed the result we are waiting for. This cannot deadlock because, as
-  mentioned before, query invocations form a DAG. Some thread will always make
+  mentioned before, query invocations form a DAG. Some threads will always make
   progress.
 
 ## Rustdoc
 
-As of <!-- date: 2021-09--> September 2021, there are still a number of steps 
+As of <!-- date: 2022-05--> May 2022, there are still a number of steps 
 to complete before rustdoc rendering can be made parallel. More details on
 this issue can be found [here][parallel-rustdoc].
 
 ## Current Status
 
-As of <!-- date: 2021-07 --> July 2021, work on explicitly parallelizing the
+As of <!-- date: 2022-05 --> May 2022, work on explicitly parallelizing the
 compiler has stalled. There is a lot of design and correctness work that needs
 to be done.
 
@@ -76,7 +76,7 @@ These are the basic ideas in the effort to make `rustc` parallel:
 
 [`rayon`]: https://crates.io/crates/rayon
 
-As of <!-- date: 2021-02 --> February 2021, much of this effort is on hold due
+As of <!-- date: 2022-05 --> May 2022, much of this effort is on hold due
 to lack of manpower. We have a working prototype with promising performance
 gains in many cases. However, there are two blockers: