Update readme.md and adjust image:alt functionality

Author: ItayZiv

README.md

@@ -62,36 +62,37 @@ ogp_custom_meta_tags = [
 ```
 
 ## Per Page Overrides
-[Field lists](https://www.sphinx-doc.org/en/master/usage/restructuredtext/field-lists.html) are used to allow you to override certain settings on each page.
+[Field lists](https://www.sphinx-doc.org/en/master/usage/restructuredtext/field-lists.html) are used to allow you to override certain settings on each page and set unsupported arbitrary OpenGraph tags.
 
 Make sure you place the fields at the very start of the document such that Sphinx will pick them up and also won't build them into the html.
 
 ### Overrides
+These are some overrides that can be used, you can actually override any tag and field lists will always take priority.
 
-* `:ogp-description-length:`
-  * Configure the amount of characters to grab for the description of the page. If the value isn't a number it will fall back to `ogp_description_length`.
-* `:ogp-description:`
+* `:og_description_length:`
+  * Configure the amount of characters to grab for the description of the page. If the value isn't a number it will fall back to `ogp_description_length`. Note the slightly different syntax because this isn't directly an OpenGraph tag.
+* `:og:description:`
   * Lets you override the description of the page.
-* `:ogp-title:`
+* `:og:title:`
   * Lets you override the title of the page.
-* `:ogp-type:`
+* `:og:type:`
   * Override the type of the page, for the list of available types take a look at https://ogp.me/#types.
-* `:ogp-image:`
+* `:ogp:image:`
   * Set the image for the page.
-* `:ogp-image-alt:`
-  * Will be ignored if the image isn't set with the above field, if the image is set, sets the alt text for it.
+* `:ogp:image:alt:`
+  * Sets the alt text. Will be ignored if there is no image set.
 
 ### Example
 Remember that the fields **must** be placed at the very start of the file. You can verify Sphinx has picked up the fields if they aren't shown in the final html file.
 
 ```rst
-:ogp-description: New description
-:ogp-image: http://example.org/image.png
-:ogp-image-alt: Example Image
+:og:description: New description
+:og:image: http://example.org/image.png
+:og:image:alt: Example Image
 ```
 
-## Arbitrary Tags
-Additionally, you can use field lists to add any arbitrary OpenGraph tag not supported by the extension. The syntax for arbitrary tags is `:og:tag: content`. For Example:
+### Arbitrary Tags
+Additionally, you can use field lists to add any arbitrary OpenGraph tag not supported by the extension. The syntax for arbitrary tags is the same with `:og:tag: content`. For Example:
 
 ```rst
 :og:video: http://example.org/video.mp4

sphinxext/opengraph/__init__.py

@@ -109,11 +109,11 @@ def get_tags(
         ogp_use_first_image = False
         ogp_image_alt = fields.get("og:image:alt")
         fields.pop("og:image", None)
-        fields.pop("og:image:alt", None)
     else:
         image_url = config["ogp_image"]
         ogp_use_first_image = config["ogp_use_first_image"]
-        ogp_image_alt = config["ogp_image_alt"]
+        ogp_image_alt = fields.get("og:image:alt", config["ogp_image_alt"])
+    fields.pop("og:image:alt", None)
 
     if ogp_use_first_image:
         first_image = doctree.next_node(nodes.image)