Edits/corrections to Parental Leave policy

[rdpiazza]

• Removed "human readable" as a description of summary, because that discourages people from reading the (surprisingly good!) document from OPM
• Added reference to adoption
• Changed "mother" to "birth mother" to use inclusive language
• Removed "incapacitation" and replaced with "medical recovery" because the switch in terminology was confusing
• Changed "After parents exhaust their annual and sick leave" to "If a parent does not have sufficient sick leave" because you do not need to exhaust annual leave before advancing sick leave.
• Added section addressing options for after the medically necessary leave period, since otherwise it sounds like you can only take 6-8 weeks.
• Supervisor approval is not required for FMLA
• Unpaid leave that is not FMLA leave is available for people who haven't been here a year

[coreycaitlin]

This content also exists in the handbook here (https://handbook.18f.gov/leave-telework-and-virtual-worker/#parental-leave), and the new, integrated Join 18F page links to that resource. We should make sure these edits are reflected in the Handbook before closing this PR.