Improve inline documentation

- [ ] fieldmanager.php
- [ ] php/class-fieldmanager-autocomplete.php
- [ ] php/class-fieldmanager-checkbox.php
- [ ] php/class-fieldmanager-checkboxes.php
- [ ] php/class-fieldmanager-datepicker.php
- [ ] php/class-fieldmanager-draggablepost.php
- [ ] php/class-fieldmanager-field.php
- [ ] php/class-fieldmanager-grid.php
- [ ] php/class-fieldmanager-group.php
- [ ] php/class-fieldmanager-hidden.php
- [ ] php/class-fieldmanager-link.php
- [ ] php/class-fieldmanager-media.php
- [ ] php/class-fieldmanager-options.php
- [ ] php/class-fieldmanager-password.php
- [ ] php/class-fieldmanager-radios.php
- [ ] php/class-fieldmanager-richtextarea.php
- [ ] php/class-fieldmanager-select.php
- [ ] php/class-fieldmanager-textarea.php
- [ ] php/class-fieldmanager-textfield.php
- [ ] php/context/class-fieldmanager-context-page.php
- [ ] php/context/class-fieldmanager-context-post.php
- [ ] php/context/class-fieldmanager-context-quickedit.php
- [ ] php/context/class-fieldmanager-context-storable.php
- [ ] php/context/class-fieldmanager-context-submenu.php
- [ ] php/context/class-fieldmanager-context-term.php
- [ ] php/context/class-fieldmanager-context-user.php
- [ ] php/context/class-fieldmanager-context.php
- [ ] php/datasource/class-fieldmanager-datasource-post.php
- [ ] php/datasource/class-fieldmanager-datasource-term.php
- [ ] php/datasource/class-fieldmanager-datasource-user.php
- [ ] php/datasource/class-fieldmanager-datasource.php
- [ ] php/util/class-fieldmanager-util-term-meta.php
- [ ] php/util/class-fieldmanager-util-validation.php