doc: extensions: boards: add search tips admonition

As the search form is becoming richer, provide some guidance on how to
filter the list of supported boards and what to do when no results are
found.

Signed-off-by: Benjamin Cabé <[email protected]>

boards/index.rst

@@ -13,8 +13,20 @@ Shields are hardware add-ons that can be stacked on top of a board to add extra
 functionality. They are listed separately from boards, towards :ref:`the end of
 this page <boards-shields>`.
 
-Use the interactive search form below to quickly navigate through the list of
-supported boards.
+.. admonition:: Search Tips
+   :class: dropdown
+
+   * Use the form below to filter the list of supported boards. If a field is left empty, it will
+     not be used in the filtering process.
+
+   * A board must meet **all** criteria selected across different fields. For example, if you select
+     both a vendor and an architecture, only boards that match both will be displayed. Within a
+     single field, selecting multiple options (such as two architectures) will show boards matching
+     **either** option.
+
+   * Can't find your exact board? Don't worry! If a similar board with the same or a closely related
+     MCU exists, you can use it as a :ref:`starting point <create-your-board-directory>` for adding
+     support for your own board.
 
 .. toctree::
    :maxdepth: 2

doc/_extensions/zephyr/domain/static/js/board-catalog.js

@@ -76,7 +76,7 @@ function fillSocSocSelect(families, series = undefined, selectOnFill = false) {
 
   families = families?.length ? families : Object.keys(socs_data);
   series = series?.length ? series : families.flatMap(f => Object.keys(socs_data[f]));
-  matchingSocs = families.flatMap(f => series.flatMap(s => socs_data[f][s] || []));
+  matchingSocs = [...new Set(families.flatMap(f => series.flatMap(s => socs_data[f][s] || [])))];
 
   socSocSelect.innerHTML = "";
   matchingSocs.sort().forEach((soc) => {