doc: mark TgaImageConverter as having no known caveats.

While StbImageConverter has now a minor caveat as it produces larger
files for no apparent reason.

doc/file-formats.dox

@@ -407,13 +407,13 @@ Derived from @ref Trade::AbstractImageConverter.
 <th rowspan="2">Truevision TGA<br/>(`*.tga`, `*.vda`, `*.icb`, `*.vst`)</th>
 <td rowspan="2">`TgaImageConverter`</td>
 <td>@ref Trade::TgaImageConverter "TgaImageConverter"</td>
-<td class="m-text-center m-success">@ref Trade-TgaImageConverter-behavior "minor"</td>
+<td class="m-text-center m-dim">@ref Trade-TgaImageConverter-behavior "none"</td>
 <td class="m-text-center">@m_span{m-text m-dim} none @m_endspan </td>
 <td class="m-text-center"></td>
 </tr>
 <tr>
 <td>@ref Trade::StbImageConverter "StbImageConverter"</td>
-<td class="m-text-center m-dim">@ref Trade-StbImageConverter-behavior "none"</td>
+<td class="m-text-center m-success">@ref Trade-StbImageConverter-behavior-tga-rle "minor"</td>
 <td class="m-text-center m-primary">bundled</td>
 <td class="m-text-center m-primary">public&nbsp;domain</td>
 </tr>