Remove the string about level 1C in i.sentinel.mask

I also moved the requirement for BOA input data upwards to make it clear from the beginning that atmospherically corrected S2 images are needed.

grass7/imagery/i.sentinel/i.sentinel.mask/i.sentinel.mask.html

@@ -1,7 +1,12 @@
 <h2>DESCRIPTION</h2>
 
 <em>i.sentinel.mask</em> allows to automatically identify clouds and their
-shadows in Sentinel-2 images (level 1C and 2A).
+shadows in Sentinel-2 images. 
+The algorithm works on reflectance values (Bottom of Atmosphere Reflectance)
+therefore, the atmospheric correction has to be applied to all input bands (see
+<a href="i.sentinel.preproc.html">i.sentinel.preproc</a> or <a
+href="i.atcorr.html">i.atcorr</a>) (level 1C and 2A).
+
 <p>
 The following figures show the difference between the standard cloud mask as
 provided in Sentinel-2 SAFE products and the cloud detection results obtained
@@ -62,12 +67,6 @@ <h2>DESCRIPTION</h2>
 </center>
 -->
 
-<p>
-The algorithm works on reflectance values (Bottom of Atmosphere Reflectance)
-therefore, the atmospheric correction has to be applied to all input bands (see
-<a href="i.sentinel.preproc.html">i.sentinel.preproc</a> or <a
-href="i.atcorr.html">i.atcorr</a>)
-
 <p>
 All necessary input bands (blue, green, red, nir, nir8a, swir11, swir12) must
 be imported in GRASS and specified one by one or using an input text file. The