documentation

Pull request #3997: Feature/ZBXNEXT-943 7.5 D

Merge in WEB/documentation from feature/ZBXNEXT-943-7.5-D to release/8.0

* commit 'e48ed6297d58a375ef268bbc7fdb55978ae924cb':
  [ZBXNEXT-943] minor typo fix
  [ZBXNEXT-943] fixed field name
  [ZBXNEXT-943] fixed formatting
  [ZBXNEXT-943] updated API documentation
  [ZBXNEXT-943] updated image
  [ZBXNEXT-943] updated documentation with negative Y-axis graphing option
  [ZBXNEXT-943] updated documentation with negative Y-axis graphing option