validate -h corrections/clarifications

[tbarnes4]

Describe the bug Per discussion in #288, it was identified to update some descriptions in the validate tool help (-h flag):

1. -R,--rule flag description text should be updated to require or strongly suggest the user specify which validation rules they are using. The auto-detect may not function as (at least this) user expects. On the documentation page it states how auto-detect works:

If the -R flag is not specified on the command-line, the default behavior is to use the pds4.label rule for target file inputs and to use the pds4.folder rule for target directory inputs.

2. -t,--target flag description should be removed per @jordanpadams in https://github.com/NASA-PDS/validate/issues/288#issuecomment-768312426. I am not sure how this should be changed.

To Reproduce To see the help description: validate -h

** Version of Software Used** gov.nasa.pds:validate Version 1.24.0 Release Date: 2020-09-08 22:33:13

Test Data / Additional context Ref: #288