func (s *WikisService) EditWikiPage(pid interface{}, slug string, opt *EditWikiPageOptions, options ...OptionFunc) (*Wiki, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/wikis/%s", pathEscape(project), url.PathEscape(slug))
	req, err := s.client.NewRequest("PUT", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	w := new(Wiki)
	resp, err := s.client.Do(req, w)
	if err != nil {
		return nil, resp, err
	}
	return w, resp, err
}
EditWikiPage Updates an existing wiki page. At least one parameter is
required to update the wiki page.

GitLab API docs:
https://docs.gitlab.com/ce/api/wikis.html#edit-an-existing-wiki-page
package gitlab
import (
	"fmt"
	"net/url"
)
type WikisService struct {
	client *Client
}
type WikiFormat string
const (
	WikiFormatMarkdown WikiFormat = "markdown"
	WikiFormatRFoc     WikiFormat = "rdoc"
	WikiFormatASCIIDoc WikiFormat = "asciidoc"
)
type Wiki struct {
	Content string     `json:"content"`
	Format  WikiFormat `json:"format"`
	Slug    string     `json:"slug"`
	Title   string     `json:"title"`
}
func (w Wiki) String() string {
	return Stringify(w)
}
type ListWikisOptions struct {
	WithContent *bool `url:"with_content,omitempty" json:"with_content,omitempty"`
}
func (s *WikisService) ListWikis(pid interface{}, opt *ListWikisOptions, options ...OptionFunc) ([]*Wiki, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/wikis", pathEscape(project))
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var w []*Wiki
	resp, err := s.client.Do(req, &w)
	if err != nil {
		return nil, resp, err
	}
	return w, resp, err
}
func (s *WikisService) GetWikiPage(pid interface{}, slug string, options ...OptionFunc) (*Wiki, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/wikis/%s", pathEscape(project), url.PathEscape(slug))
	req, err := s.client.NewRequest("GET", u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	var w *Wiki
	resp, err := s.client.Do(req, &w)
	if err != nil {
		return nil, resp, err
	}
	return w, resp, err
}
type CreateWikiPageOptions struct {
	Content *string `url:"content" json:"content"`
	Title   *string `url:"title" json:"title"`
	Format  *string `url:"format,omitempty" json:"format,omitempty"`
}
func (s *WikisService) CreateWikiPage(pid interface{}, opt *CreateWikiPageOptions, options ...OptionFunc) (*Wiki, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/wikis", pathEscape(project))
	req, err := s.client.NewRequest("POST", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	w := new(Wiki)
	resp, err := s.client.Do(req, w)
	if err != nil {
		return nil, resp, err
	}
	return w, resp, err
}
type EditWikiPageOptions struct {
	Content *string `url:"content" json:"content"`
	Title   *string `url:"title" json:"title"`
	Format  *string `url:"format,omitempty" json:"format,omitempty"`
}