Describe the behavior of floating window support in document

doc/LanguageClient.txt

@@ -63,6 +63,12 @@ accessed by regular quickfix/location list operations.
 To use the language server with Vim's formatting operator |gq|, set 'formatexpr': >
     set formatexpr=LanguageClient#textDocument_rangeFormatting_sync()
 <
+If you're using Neovim 0.4.0 or later, |LanguageClient#textDocument_hover()|
+opens documentation in a floating window. The window is automatically closed
+when you move the cursor. Or calling |LanguageClient#textDocument_hover()|
+again just after opening the floating window moves the cursor into the window.
+It is useful when documentation is longer and you need to scroll down or you
+want to yank some text in the documentation.
 
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 2. Configuration                                 *LanguageClientConfiguration*