[MAINT] Improve 'try_get_value' readme
[MAINT] Improve 'try_get_value' readme

puppetlabs/puppetlabs-stdlib,fuel-infra/puppetlabs-stdlib,HelenCampbell/puppetlabs-stdlib,yadavnikhil/puppetlabs-stdlib,DavidS/puppetlabs-stdlib,jbondpdx/puppetlabs-stdlib