docs: improve the description of the 'tabstospaces' option

It never mentioned to how many spaces a tab would be converted.

doc/nano.1

@@ -118,7 +118,8 @@ by setting the options \fBtitlecolor\fP, \fBstatuscolor\fP, \fBkeycolor\fP,
 nanorc file.  See \fBnanorc\fR(5).
 .TP
 .BR \-E ", " \-\-tabstospaces
-Convert typed tabs to spaces.
+Convert each typed tab to spaces -- to the number of spaces
+that a tab at that position would take up.
 .TP
 .BR \-F ", " \-\-multibuffer
 Read a file into a new buffer by default.

doc/nano.texi

@@ -195,7 +195,8 @@ nanorc file.  @xref{@code{set functioncolor}} for details.
 
 @item -E
 @itemx --tabstospaces
-Convert typed tabs to spaces.
+Convert each typed tab to spaces --- to the number of spaces
+that a tab at that position would take up.
 
 @item -F
 @itemx --multibuffer
@@ -1095,7 +1096,8 @@ Use a tab size of @var{number} columns.  The value of @var{number} must be
 greater than 0.  The default value is @t{8}.
 
 @item set tabstospaces
-Convert typed tabs to spaces.
+Convert each typed tab to spaces --- to the number of spaces
+that a tab at that position would take up.
 
 @item set titlecolor [bold,][italic,]@var{fgcolor},@var{bgcolor}
 Use this color combination for the title bar.

doc/nanorc.5

@@ -345,7 +345,8 @@ Use a tab size of \fInumber\fR columns.  The value of \fInumber\fP must be
 greater than 0.  The default value is \fB8\fR.
 .TP
 .B set tabstospaces
-Convert typed tabs to spaces.
+Convert each typed tab to spaces -- to the number of spaces
+that a tab at that position would take up.
 .TP
 .B set titlecolor \fR[\fBbold,\fR][\fBitalic,\fR]\fIfgcolor\fB,\fIbgcolor\fR
 Use this color combination for the title bar.

doc/sample.nanorc.in

@@ -174,7 +174,7 @@
 ## Use this tab size instead of the default; it must be greater than 0.
 # set tabsize 8
 
-## Convert typed tabs to spaces.
+## Convert each typed tab to the fitting number of spaces.
 # set tabstospaces
 
 ## Snip whitespace at the end of lines when justifying or hard-wrapping.