Simplify namespace guidance for translators

What does this MR do and why?

Describe in detail what your merge request does and why.

As part of my experience learning how to contribute translations to GitLab, documented here, I found the namespace guidance confusing at https://docs.gitlab.com/ee/development/i18n/translation.html#namespaced-strings. I'm hoping that this MR makes it easier by:

• Removing the use of the word "prepended" with something more commonly used.
• Clarifying what part of the string is the namespace in the example given.
• Providing a, hopefully accurate, tip on how to quickly identify namespaces in strings (e.g., added as somethingsomething| to the beginning of the string.

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