[FEATURE]Add help command to PPL for a descriptive of grammar

[YANG-DB]

Is your feature request related to a problem?
A simple self contained way to get the PPL grammar description and usage example, this would also allow new users to get basic understanding of the language and its functionality

What solution would you like?
The next example reviews the trendline command:

>> help trendline

DESCRIPTION
    Computes moving averages of fields: simple moving average (SMA) and weighted moving average (WMA). The output is written to a new field.

SYNTAX
    trendline <trendtype>(<period>, <field>) [AS <newfield>] [, ...]

ARGUMENTS
    trendtype    (Required) The type of trend to compute. Supported types:
                 sma: Simple Moving Average
                 wma: Weighted Moving Average

    period       (Required) An integer between 2 and 10000 representing the period over which to compute the trend.

    field        (Required) The name of the field on which to calculate the trend.

    newfield     (Optional) The name of the new field to store the result. If not specified, defaults to <trendtype><period>(<field>).

USAGE EXAMPLE
    source=cpu_metrics 
    | where host='webserver01' 
    | trendline sma(5, cpu_usage) AS smooth_cpu, wma(10, memory_usage) AS mem_trend 
    | stats avg(smooth_cpu), max(mem_trend) by date_histogram(timestamp, '1h')