Add Auto-Checking Minimum Size preference

- add new preference and description
- add Note for Find-Fill to explain that it's 1024-character limit is separate from the new setting

closes #581

Author: pryrt

content/docs/preferences.md

@@ -292,8 +292,13 @@ Affects how the text is formatted when sent to the printer.
 Affects the operations found in the **Find** dialog tabs.
 
 * **When Find Dialog is Invoked**
+    * `Minimum Size for Auto-Checking 'In selection': _____`: Integer value used for deciding whether to automatically checkmark **☐ In Selection**.  (new to v8.5.8)
+        - When a search function is initiated while a stream selection is active in the editor, the **☐ In Selection** option will become automatically checkmarked if this setting is not `0` and the number of characters in the selection is above the specified value. Additionally, if this setting is non-zero, the **☐ In selection** option will be automatically cleared when a search is initiated with no stream selection active in the editor. When the setting is `0`, the user will entirely control the state of the **☐ In selection** checkbox (that is, the program won't change the check state at all). 
+        - The default value is `1024` characters.  
+        - Allowed values: `0` characters is the smallest integer value allowed.  `1024` is the largest integer value allowed. If you enter an integer value greater than `1024` then the field value will change to `1024` immediately. If you attempt to enter a non-digit character (like a decimal point or a letter), the field will pop up an error message indicating that only digits are allowed to be entered.
     * `☐ Fill Find Field with Selected Text`: When checkmarked, invoking the Find Dialog will fill the **Find What** field with the currently-selected text; when not checkmarked, invoking the Find Dialog will not change the contents of the **Find What** field (reworded in v8.4.5; previously was `Don't fill find field...`)
         * `☐ Select Word Under Caret when Nothing Selected`: When checkmarked, invoking the Find Dialog with no active selection will fill the **Find What** field with the text under the caret; when not checkmarked, invoking the Find Dialog with no active selection will not change the contents of the **Find What** field; if the option above is not checkmarked, this option will be un-checkmarked and cannot be toggled.  (new to v8.4.5)
+    * _Note_: The fill will only occur if the size of the active selection is 1024 characters or less. This is a fixed value and is unrelated to the `Minimum Size for Auto-Checking 'In selection'` value.
 * `☐ Use Monospaced font in Find dialog (Need to restart Notepad++)`: Changes the font from standard proportional font to a monospaced font in the text boxes in the **Find** dialog; requires restarting Notepad++ to change.
 * `☐ Find dialog remains open after search that outputs to results window`: Successful file-level searches cause the **Find** window to close; selecting this option keeps the window open always.
 * `☐ Confirm Replace All in All Opened Documents`: When checked, **Replace All in All Opened Documents** will ask for confirmation.