PHPDocs of resolve_image_location are partially inaccurate

During testing of MDL-58485, mudrd8mz correctly pointed out that the phpdocs of the \theme_config::resolve_image_location function (particularly, for the parameter $svg are inaccurate:

@param bool $svg If set to true SVG images will also be looked for.

However, implementation of that function explicitly handles the case that $svg is null, which results in a fallback to browser support of SVGs. Therefore, docs should be updated to

@param bool|null $svg Should SVG images be also looked for, null for auto-detection of browser support.

... I'll prepare a patch.