document select.

DOC.md

@@ -1867,6 +1867,20 @@ The first argument has to be a table, which defines at least `trig` and
     * `match_captures`, `string|string[]`: The capture(s) to use for determining
       the actual prefix (so the node that should be immediately in front of the
       trigger). This defaults to just `"prefix"`.
+    * `select`, `string?|fun(): LuaSnip.extra.MatchSelector`: Since there may be
+      multiple matching captures in front of the cursor, there has to be some
+      way to select the node that will actually be used.  
+      If this is a string, it has to be one of "any", "shortest", or "longest",
+      which mean that any, the shortest, or the longest match is used.  
+      If it is a function, it must return a table with two fields, `record` and
+      `retrieve`. `record` is called with a TSMatch and a potential node for the
+      ts-match, and may return `true` to abort the selection-procedure.
+      `retrieve` must return either a TSMatch-TSNode-tuple (which is used as the
+      match) or `nil`, to signify that there is no match.  
+      `lua/luasnip/extras/_treesitter.lua` contains the table
+      `builtin_tsnode_selectors`, which contains the implementations for
+      any/shortest/longest, which can be used as examples for more complicated
+      custom-selectors.
 
 The text of the matched node can be accessed as `snip.env.LS_TSMATCH`.  
 The text of the nodes returned as `NamedTSMatch` can be accessed as