[Doc]Good to add relevant useful stats on cbstats:timings and update sample output

Often customers want to know which cbstats-timing stats should they be looking at, typically to understand their get/set timings.

The current documentation shows a generic output from timings, with no specific mention of these, which stats are most relevant https://developer.couchbase.com/documentation/server/current/cli/cbstats/cbstats-timing.html

Imo, It would be good to include

1. Which top stats to look for esp for gets and sets - 'get_cmd' and 'store_cmd'
2. Update the sample  output to include 'get_cmd' and 'store_cmd'

  get_cmd (2 total)
    4us - 8us : ( 50.00%) 1 ####################################################
    1ms - 2ms : (100.00%) 1 ####################################################

 store_cmd (15767 total)
    2us - 4us     : (  0.72%)  114
    4us - 8us     : ( 29.99%) 4614 ############################
    8us - 16us    : ( 80.52%) 7967 #################################################
    16us - 32us   : ( 91.60%) 1747 ##########
    32us - 64us   : ( 97.77%)  973 #####
    64us - 128us  : ( 98.87%)  174 #
    128us - 256us : ( 99.25%)   59
    256us - 512us : ( 99.59%)   55
    512us - 1ms   : ( 99.81%)   34
    1ms - 2ms     : ( 99.94%)   21
    2ms - 4ms     : ( 99.97%)    5
    4ms - 8ms     : ( 99.98%)    1
    8ms - 16ms    : (100.00%)    3
    Avg           : (   15us)