docs: add a clarifying note to the description of --tabstospaces

doc/nano.1

@@ -112,6 +112,7 @@ nanorc file.  See \fBnanorc\fR(5).
 .BR \-E ", " \-\-tabstospaces
 Convert each typed tab to spaces -- to the number of spaces
 that a tab at that position would take up.
+(Note: pasted tabs are not converted.)
 .TP
 .BR \-F ", " \-\-multibuffer
 Read a file into a new buffer by default.

doc/nano.texi

@@ -406,6 +406,7 @@ nanorc file.  @xref{@code{set keycolor}} for details.
 @itemx --tabstospaces
 Convert each typed tab to spaces --- to the number of spaces
 that a tab at that position would take up.
+(Note: pasted tabs are not converted.)
 
 @item -F
 @itemx --multibuffer
@@ -1140,6 +1141,7 @@ greater than 0.  The default value is @t{8}.
 @item set tabstospaces
 Convert each typed tab to spaces --- to the number of spaces
 that a tab at that position would take up.
+(Note: pasted tabs are not converted.)
 
 @item set titlecolor [bold,][italic,]@var{fgcolor},@var{bgcolor}
 Use this color combination for the title bar.

doc/nanorc.5

@@ -335,6 +335,7 @@ greater than 0.  The default value is \fB8\fR.
 .B set tabstospaces
 Convert each typed tab to spaces -- to the number of spaces
 that a tab at that position would take up.
+(Note: pasted tabs are not converted.)
 .TP
 .B set titlecolor \fR[\fBbold,\fR][\fBitalic,\fR]\fIfgcolor\fB,\fIbgcolor\fR
 Use this color combination for the title bar.