report: select: add help for creating selections

report: select: add help for creating selections

The "<lvm command> -S/--select help" shows help (including list of fields to match against):

  ...field list here including the field type name...

  Selection operands
  ------------------
    field               - Reporting field.
    number              - Non-negative integer value.
    size                - Floating point value with units specified.
    string              - Characters quoted by ' or " or unquoted.
    string list         - Strings enclosed by [ ] and elements delimited by either
                          "all items must match" or "at least one item must match" operator.
    regular expression  - Characters quoted by ' or " or unquoted.

  Selection operators
  -------------------
    Comparison operators:
        =~  - Matching regular expression.
        !~  - Not matching regular expression.
         =  - Equal to.
        !=  - Not equal to.
        >=  - Greater than or equal to.
         >  - Greater than
        <=  - Less than or equal to.
         <  - Less than.

    Logical and grouping operators:
        &&  - All fields must match
         ,  - All fields must match
        ||  - At least one field must match
         #  - At least one field must match
         !  - Logical negation
         (  - Left parenthesis
         )  - Right parenthesis
         [  - List start
         ]  - List end