You can use the built-in help apropos by typing C-ha and the word whitespace
. It will list for example
delete-trailing-whitespace M-x ... RET Delete trailing whitespace between START and END. fixup-whitespace M-x ... RET Fixup white space between objects around point.
The first deletes trailing whitespace within the current region. The second does what you want, at point. Click button 2 on the function name to get more info.
Neither probably have a default binding for you, so to execute the second you type M-x and the first few chars fixup-
and tab to complete. Add your own binding to a key for ease of use.