fixup! doc: add tuning parameters for MPI_THREAD_MULTIPLE

doc/mpich/TUNING_PARAMETERS

@@ -43,12 +43,26 @@ VCI lock and a `lockless` mode where MPI does not grab a lock and relies on the
 network provider to provide fine grain synchronization (MPICH uses the
 FI_THREAD_SAFE mode from OFI to support the `lockless` mode).
 
+A threading synchronization strategy can be selected at the configuration time
+as:
+  --enable-thread-cs=global to select coarse-grained locking, and
+  --enable-thread-cs=per-vci to select fine-grained locking.
+
+Under fine-grained locking, a specific locking mode at configuration time as:
+  --enable-ch4-mt=<mode>, mode can be one of direct, lockless and runtime.
+
+If configuration is done with --enable-ch4-mt=runtime, user can select a
+threading mode at runtime using the environment MPIR_CVAR_CH4_MT_MODEL
+(described below in Section 1.1).
+
 **1.1 CVARs (Environment Variables)**
 
 MPICH users can use the CVAR MPIR_CVAR_CH4_MT_MODEL to select at runtime among
 the threading synchronization models, `global`, `direct`, or `lockless`. If the
 CVAR is not set, the default model is `direct`.
 
+e.g., export MPIR_CVAR_CH4_MT_MODEL=direct
+
 Users can specify the number of VCIs to use by setting the environment variable
 `MPIR_CVAR_CH4_NUM_VCIS=<number_vcis>`, where the value `<num_vcis>` should be
 less or equal than the number of network contexts provided by the OFI provider.
@@ -58,6 +72,8 @@ receive contexts (max_ep_rx_ctx) it supports. MPICH uses the minimum of
 max_ep_tx_ctx and max_ep_rx_ctx as the maximum number of VCIs it can support. If
 the system has multiple NICS, this value is the `minimum across all the NICS`.
 
+e.g., export MPIR_CVAR_CH4_NUM_VCIS=16
+
 When multiple VCIs are used, MPICH tries to make per-vci progress to avoid lock
 contention inside the progress engine. However, it also progresses all VCIs,
 i.e., make global progress, once in a while to avoid potential deadlocks. Such