Document Ferret engine fulltext search features

Summary: Ref T12819. Adds some documentation for `-term`, `~term`, `title:term`, etc.

Test Plan: Read documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18592

src/docs/user/userguide/search.diviner

@@ -123,3 +123,53 @@ Another useful function is the `viewer()` function, which works as though you'd
 typed your own username when you run the query. However, if you send the query
 to someone else, it will show results for //their// username when they run it.
 This can be particularly useful when creating dashboard panels.
+
+
+Fulltext Search
+===============
+
+Global search and some applications provide **fulltext search**. In
+applications, this is a field called {nav Query}.
+
+Fulltext search allows you to search the text content of objects and supports
+some special syntax. These features are supported:
+
+  - Substring search with `~platypus`.
+  - Field search with `title:platypus`.
+  - Filtering out matches with `-platypus`.
+  - Quoted terms with `"platypus attorney"`.
+  - Combining features with `title:~"platypus attorney"`.
+
+See below for more detail.
+
+**Substrings**: Normally, query terms are searched for as words, so searching
+for `read` won't find documents which only contain the word `threaded`, even
+though "read" is a substring of "threaded". With the substring operator, `~`,
+you can search for substrings instead: the query `~read` will match documents
+which contain that text anywhere, even in the middle of a word.
+
+**Quoted Terms**: When you search for multiple terms, documents which match
+each term will be returned, even if the terms are not adjacent in the document.
+For example, the query `void star` will match a document titled `A star in the
+void`, because it matches both `void` and `star`. To search for an exact
+sequence of terms, quote them: `"void star"`. This query will only match
+documents which use those terms as written.
+
+**Stemming**: Searching for a term like `rearming` will find documents which
+contain variations of the word, like `rearm`, `rearms`, and `rearmed`. To
+search for an an exact word, quote the term: `"rearming"`.
+
+**Field Search**: By default, query terms are searched for in the title, body,
+and comments. If you only want to search for a term in titles, use `title:`.
+For example, `title:platypus` only finds documents with that term in the
+title. This can be combined with other operators, for example `title:~platypus`
+or `title:"platypus attorney"`. These scopes are also supported:
+
+  - `title:...` searches titles.
+  - `body:...` searches bodies (descriptions or summaries).
+  - `core:...` searches titles and bodies, but not comments.
+  - `comments:...` searches only comments.
+
+**Filtering Matches**: You can remove documents which match certain terms from
+the result set with `-`. For example: `platypus -mammal`. Documents which match
+negated terms will be filtered out of the result set.