Improve a bunch of help texts

notebooks/tests/animate.ipynb

notebooks/tests/assert.ipynb

notebooks/tests/eval.ipynb

notebooks/tests/help.ipynb

notebooks/tests/let.ipynb

notebooks/tests/load_cell.ipynb

notebooks/tests/load_file.ipynb

notebooks/tests/pref.ipynb

notebooks/tests/time.ipynb

notebooks/tests/trace.ipynb

notebooks/tests/type.ipynb

src/main/java/de/prob2/jupyter/commands/AssertCommand.java

src/main/java/de/prob2/jupyter/commands/BrowseCommand.java

src/main/java/de/prob2/jupyter/commands/BsymbCommand.java

src/main/java/de/prob2/jupyter/commands/EvalCommand.java

src/main/java/de/prob2/jupyter/commands/ExecCommand.java

src/main/java/de/prob2/jupyter/commands/FindCommand.java

src/main/java/de/prob2/jupyter/commands/GotoCommand.java

src/main/java/de/prob2/jupyter/commands/HelpCommand.java

src/main/java/de/prob2/jupyter/commands/LetCommand.java