docs: update upgrade guide with more details about mapstructure

Author: sagikazarmark

UPGRADE.md

@@ -85,14 +85,24 @@ v := viper.NewWithOptions(
 
 ### BREAKING: "github.com/mitchellh/mapstructure" depedency replaced
 
-`mapstructure` package import has changed ([#1723](https://github.com/spf13/viper/pull/1723)) and code like take does not compile anymore
+The original [mapstructure](https://github.com/mitchellh/mapstructure) has been [archived](https://github.com/mitchellh/mapstructure/issues/349) and was replaced with a [fork](https://github.com/go-viper/mapstructure) maintained by Viper ([#1723](https://github.com/spf13/viper/pull/1723)).
+
+As a result, the package import path needs to be changed in cases where `mapstructure` is directly referenced in your code.
+
+For example, when providing a custom decoder config:
+
 ```go
 err := viper.Unmarshal(&appConfig, func(config *mapstructure.DecoderConfig) {
 	config.TagName = "yaml"
 })
 ```
 
-To fix this issue, replace all instances of imports [`"github.com/mitchellh/mapstructure"`](https://github.com/mitchellh/mapstructure) with [`"github.com/go-viper/mapstructure/v2"`](https://github.com/go-viper/mapstructure/).
+The change is fairly straightforward, just replace all occurrences of the import path `github.com/mitchellh/mapstructure` with `github.com/go-viper/mapstructure/v2`:
+
+```diff
+- import "github.com/mitchellh/mapstructure"
++ import "github.com/go-viper/mapstructure/v2"
+```
 
 ### BREAKING: HCL, Java properties, INI removed from core