More updates to Text chapter of Emacs manual.

More updates to Text chapter of Emacs manual.

* text.texi (Nroff Mode): Mention what nroff is.
(Text Based Tables, Table Recognition): Don't say "Table mode"
since it's not a major or minor mode.
(Text Based Tables): Reduce the size of the example.
(Table Definition): Clarify definitions.
(Table Creation): Add key table.
(Cell Commands): Use kbd for commands.
(Table Rows and Columns): Combine nodes Row Commands and Column
Commands.
(Fixed Width Mode): Node deleted; contents moved to parent.
(Table Conversion): Shorten example.
(Measuring Tables): Merge into Table Misc.