Inside of your loop, you have the key, which you can use to retrieve the value from the Map:

for (String key: mss1.keySet()) {
    System.out.println(key + ": " + mss1.get(key));
}

Just some general notes:

1. sfdcfox's answer is, as always, brilliant. Using a map like that isn't a great idea, and I'd go so far as to say it's a bad idea.
2. I like your desire to let other developers know what the parameters are, but I think the right way to solve this is with proper documentation. Apex-recipes (https://github.com/trailheadapps/apex-recipes) uses the npm package "@cparra/apexdocs" to generate standardized markdown files for classes. The per-method documentation block looks like this:

/**

• @description The doLoad method is required by the CacheBuilder interface
• This method needs to return an Object - and that's the key to the Cache
• builder interface - You must have this method return a single Object that
• is either calculated by this method, or returned from the Cache by the
• key
• @param key String used to help generate the Cache Key
• @return Object This object should be casted at the call location
• @group Platform Cache Recipes
• @see CanTheUser
• @example Account[] accounts = (Account[]) Cache.Session.get(PlatformCacheBuilderRecipes.class, 'myAccounts') */ public Object doLoad(String key) {

So that's not rendering great, but those bullet points are *'s

Apexdocs understands certain tags, like @description and when run it generates docs like this markdown.

---
layout: default
---
# PlatformCacheBuilderRecipes class

demonstrates how to use the Cache.CacheBuilder Interface

---
## Methods
### `doLoad(String key)` → `Object`

The doLoad method is required by the CacheBuilder interface This method needs to return an Object - and that's the key to the Cache builder interface - You must have this method return a single Object that is either calculated by this method, or returned from the Cache by the key

#### Parameters
|Param|Description|
|-----|-----------|
|`key` |    String used to help generate the Cache Key |

#### Return

**Type**

Object

**Description**

`Object` This object should be casted at the call location

#### Example
```java
Account[] accounts = (Account[]) Cache.Session.get(PlatformCacheBuilderRecipes.class, 'myAccounts')
```

---

Generating and making this kind of documentation available makes it easy for your third party developers to consume your methods, and readily identify what order the parameters are in.

You can also use google-collections (guava) Joiner class if you want to customize the print format

Joiner.on(",").withKeyValueSeparator("=").join(map);