gui: Add (?d) and (?i) to ignores guide

gui/default/assets/lang/lang-en.json

@@ -158,6 +158,8 @@
    "Please consult the release notes before performing a major upgrade.": "Please consult the release notes before performing a major upgrade.",
    "Please set a GUI Authentication User and Password in the Settings dialog.": "Please set a GUI Authentication User and Password in the Settings dialog.",
    "Please wait": "Please wait",
+   "Prefix indicating that the file can be deleted if preventing directory removal": "Prefix indicating that the file can be deleted if preventing directory removal",
+   "Prefix indicating that the pattern should be matched without case sensitivity": "Prefix indicating that the pattern should be matched without case sensitivity",
    "Preview": "Preview",
    "Preview Usage Report": "Preview Usage Report",
    "Quick guide to supported patterns": "Quick guide to supported patterns",

gui/default/syncthing/folder/editIgnoresModalView.html

@@ -7,6 +7,8 @@
 
     <p class="small"><span translate>Quick guide to supported patterns</span> (<a href="https://docs.syncthing.net/users/ignoring.html" target="_blank" translate>full documentation</a>):</p>
     <dl class="dl-horizontal dl-narrow small">
+      <dt><code>(?d)</code></dt> <dd><b><span translate>Prefix indicating that the file can be deleted if preventing directory removal</span></b></dd>
+      <dt><code>(?i)</code></dt> <dd><span translate>Prefix indicating that the pattern should be matched without case sensitivity</span></dd>
       <dt><code>!</code></dt> <dd><span translate>Inversion of the given condition (i.e. do not exclude)</span></dd>
       <dt><code>*</code></dt> <dd><span translate>Single level wildcard (matches within a directory only)</span></dd>
       <dt><code>**</code></dt> <dd><span translate>Multi level wildcard (matches multiple directory levels)</span></dd>