Add new --healthcheck-dial-concurrency flag from pr #15053 (#1696)

Signed-off-by: Tim Vaillancourt <tvaillancourt@slack-corp.com>

content/en/docs/20.0/reference/programs/vtcombo/_index.md

@@ -181,6 +181,7 @@ vtcombo [flags]
       --grpc_server_keepalive_timeout duration                           After having pinged for keepalive check, the server waits for a duration of Timeout and if no activity is seen even after that the connection is closed. (default 10s)
       --grpc_use_effective_callerid                                      If set, and SSL is not used, will set the immediate caller id from the effective caller id's principal.
       --health_check_interval duration                                   Interval between health checks (default 20s)
+      --healthcheck-dial-concurrency int                                 Maximum concurrency of new healthcheck connections. This should be less than the golang max thread limit of 10000. (default 1024)
       --healthcheck_retry_delay duration                                 health check retry delay (default 2ms)
       --healthcheck_timeout duration                                     the health check timeout period (default 1m0s)
       --heartbeat_enable                                                 If true, vttablet records (if master) or checks (if replica) the current time of a replication heartbeat in the sidecar database's heartbeat table. The result is used to inform the serving state of the vttablet via healthchecks.

content/en/docs/20.0/reference/programs/vtctld/_index.md

@@ -101,6 +101,7 @@ vtctld \
       --grpc_server_keepalive_enforcement_policy_permit_without_stream   gRPC server permit client keepalive pings even when there are no active streams (RPCs)
       --grpc_server_keepalive_time duration                              After a duration of this time, if the server doesn't see any activity, it pings the client to see if the transport is still alive. (default 10s)
       --grpc_server_keepalive_timeout duration                           After having pinged for keepalive check, the server waits for a duration of Timeout and if no activity is seen even after that the connection is closed. (default 10s)
+      --healthcheck-dial-concurrency int                                 Maximum concurrency of new healthcheck connections. This should be less than the golang max thread limit of 10000. (default 1024)
   -h, --help                                                             help for vtctld
       --jaeger-agent-host string                                         host and port to send spans to. if empty, no tracing will be done
       --keep_logs duration                                               keep logs for this long (using ctime) (zero to keep forever)

content/en/docs/20.0/reference/programs/vtgate/_index.md

@@ -114,6 +114,7 @@ vtgate \
       --grpc_server_keepalive_time duration                              After a duration of this time, if the server doesn't see any activity, it pings the client to see if the transport is still alive. (default 10s)
       --grpc_server_keepalive_timeout duration                           After having pinged for keepalive check, the server waits for a duration of Timeout and if no activity is seen even after that the connection is closed. (default 10s)
       --grpc_use_effective_callerid                                      If set, and SSL is not used, will set the immediate caller id from the effective caller id's principal.
+      --healthcheck-dial-concurrency int                                 Maximum concurrency of new healthcheck connections. This should be less than the golang max thread limit of 10000. (default 1024)
       --healthcheck_retry_delay duration                                 health check retry delay (default 2ms)
       --healthcheck_timeout duration                                     the health check timeout period (default 1m0s)
   -h, --help                                                             help for docgen