Merge branch 'minor_doc_updates' into 'master'

Minor doc updates

## What does this MR do?

Adds to and clarifies some documentation. Specifically, when using `gdb` to debug you need to detach and exit when done. Next, this clarifies the type of file locking that we require.

Thanks to @northrup for the details about mandatory vs. advisory file locking.

Note: These were two really small changes so I combined in to one merge request. However, I left the commits separate for clarity.

See merge request !4421

doc/administration/high_availability/nfs.md

@@ -2,8 +2,8 @@
 
 ## Required NFS Server features
 
-**File locking**: GitLab **requires** file locking which is only supported
-natively in NFS version 4. NFSv3 also supports locking as long as
+**File locking**: GitLab **requires** advisory file locking, which is only
+supported natively in NFS version 4. NFSv3 also supports locking as long as
 Linux Kernel 2.6.5+ is used. We recommend using version 4 and do not
 specifically test NFSv3.
 

doc/administration/troubleshooting/sidekiq.md

@@ -150,6 +150,14 @@ To output a backtrace from all threads at once:
 apply all thread bt
 ```
 
+Once you're done debugging with `gdb`, be sure to detach from the process and
+exit:
+
+```
+detach
+exit
+```
+
 ## Check for blocking queries
 
 Sometimes the speed at which Sidekiq processes jobs can be so fast that it can