Make word completions less intrusive (#36745)

Introduce `min_words_query_len` threshold for automatic word completion
display, and set it to 3 by default.

Re-enable word completions in Markdown and Plaintext.

Release Notes:

- Introduced `min_words_query_len` threshold for automatic word
completion display, and set it to 3 by default to make them less
intrusive

docs/src/configuring-zed.md

@@ -2425,6 +2425,7 @@ Examples:
 {
   "completions": {
     "words": "fallback",
+    "words_min_length": 3,
     "lsp": true,
     "lsp_fetch_timeout_ms": 0,
     "lsp_insert_mode": "replace_suffix"
@@ -2444,6 +2445,17 @@ Examples:
 2. `fallback` - Only if LSP response errors or times out, use document's words to show completions
 3. `disabled` - Never fetch or complete document's words for completions (word-based completions can still be queried via a separate action)
 
+### Min Words Query Length
+
+- Description: Minimum number of characters required to automatically trigger word-based completions.
+  Before that value, it's still possible to trigger the words-based completion manually with the corresponding editor command.
+- Setting: `words_min_length`
+- Default: `3`
+
+**Options**
+
+Positive integer values
+
 ### LSP
 
 - Description: Whether to fetch LSP completions or not.