From https://forum.obsidian.md/t/obsidian-release-v0-8-6/4770 :
New search operator content: will search the content of the file only.
Reverse of the following ↓
The operator is missing from suggestions in search pane. The operator is not documented in Search - Obsidian Help
The operator list is currently trimmed down to avoid it from taking too much space.
Trimmed from suggestions? That makes sense. But it could still be documented. Perhaps section about operators could be extracted to separate page if it is considered too long. Or add link to page about other / advanced / less popular operators.
Yeah the documentation site does need to be updated.