Document max number of concurrent Git requests over SSH and HTTP

It was nowhere documented that, thanks to the ProjectQoSFilter [1], the
max number of concurrent Git requests over HTTP and SSH together was
limited by the sshd.threads and sshd.batchThreads settings.

This change documents this behaviour so that Gerrit admins are at least
aware of it.

Long term we can also discuss if ProjectQoSFilter still makes sense.

[1] https://gerrit.googlesource.com/gerrit/+/1a7b08e5dcd9ef6792a78f2694032e83081e59df/gerrit-pgm/src/main/java/com/google/gerrit/pgm/http/jetty/ProjectQoSFilter.java#51

Change-Id: I40393d219a44a5f8680dd3b2f8c34a2d8094d044

Documentation/config-gerrit.txt

@@ -2546,6 +2546,12 @@ By default, 5, suitable for most lower-volume traffic sites.
 Maximum number of threads to permit in the worker thread pool.
 +
 By default 25, suitable for most lower-volume traffic sites.
++
+[NOTE]
+Unless SSH daemon is disabled, see <<sshd.listenAddress, sshd.listenAddress>>,
+the max number of concurrent Git requests over HTTP and SSH together is
+defined by the <<sshd.threads, sshd.threads>> and
+<<sshd.batchThreads, sshd.batchThreads>>.
 
 [[httpd.maxQueued]]httpd.maxQueued::
 +
@@ -4143,6 +4149,10 @@ If additional requests are received while all threads are busy they
 are queued and serviced in a first-come-first-served order.
 +
 By default, 2x the number of CPUs available to the JVM.
++
+[NOTE]
+When SSH daemon is enabled then this setting also defines the max number of
+concurrent Git requests for interactive users over SSH and HTTP together.
 
 [[sshd.batchThreads]]sshd.batchThreads::
 +
@@ -4161,6 +4171,10 @@ than the total number of threads allocated in sshd.threads, then the
 value of sshd.threads is increased to accommodate the requested value.
 +
 By default is 1 on single core node, 2 otherwise.
++
+[NOTE]
+When SSH daemon is enabled then this setting also defines the max number of
+concurrent Git requests for batch users over SSH and HTTP together.
 
 [[sshd.streamThreads]]sshd.streamThreads::
 +