[doc] Complete list of theorem envs is incomplete

As reported by https://tex.stackexchange.com/q/569670, predefined theorem environments lemma, problem, solution and proof are missing from the "complete list". I hesitate to provide a PR because I cannot fix the following incomplete German part, for I cannot speak German.

https://github.com/josephwright/beamer/blob/0a94777966e0231bee06e1db98d880ea751d04d6/doc/beamerug-localstructure.tex#L518-L521

For better readability:

\subsection{Theorem Environments}
\label{section-theorems}

The \beamer\ class predefines several environments, like |theorem| or |definition| or |proof|, that you can use to typeset things like, well, theorems, definitions, or proofs.

The complete list is the following: |theorem|, |corollary|, |definition|, |definitions|, |fact|, |example|, and |examples|.

The following German block environments are also predefined: |Problem|, |Loesung|, |Definition|, |Satz|, |Beweis|, |Folgerung|, |Lemma|, |Fakt|, |Beispiel|, and |Beispiele|.

Corresponding source:
https://github.com/josephwright/beamer/blob/0a94777966e0231bee06e1db98d880ea751d04d6/base/beamerbasetheorems.sty#L109-L151

Start with doc/beamerug-localstructure.tex at the linked Theorem Environments section and compare its complete list with the predefined environments in base/beamerbasetheorems.sty. Check the German block as well; the documentation is done when both lists accurately reflect the corresponding source definitions.