add underscoreasletter config variable to process _ as a letter

src/doc/user/usermanual.html

@@ -8933,12 +8933,21 @@ for i in range(nres):
                 id=
                 "RCL.INSTALL.CONFIG.RECOLLCONF.BACKSLASHASLETTER"></a><span class="term"><code class="varname">backslashasletter</code></span></dt>
                 <dd>
-                  <p>Process backslash as normal letter This may
+                  <p>Process backslash as normal letter. This may
                   make sense for people wanting to index TeX
                   commands as such but is not of much general
                   use.</p>
                 </dd>
                 <dt><a name=
+                "RCL.INSTALL.CONFIG.RECOLLCONF.UNDERSCOREASLETTER"
+                id=
+                "RCL.INSTALL.CONFIG.RECOLLCONF.UNDERSCOREASLETTER"></a><span class="term"><code class="varname">underscoreasletter</code></span></dt>
+                <dd>
+                  <p>Process underscore as normal letter. This
+                  makes sense in so many cases that one wonders if
+                  it should not be the default.</p>
+                </dd>
+                <dt><a name=
                 "RCL.INSTALL.CONFIG.RECOLLCONF.MAXTERMLENGTH" id=
                 "RCL.INSTALL.CONFIG.RECOLLCONF.MAXTERMLENGTH"></a><span class="term"><code class="varname">maxtermlength</code></span></dt>
                 <dd>
@@ -9865,7 +9874,7 @@ for i in range(nres):
                   <p>Path for the tesseract command. This is mostly
                   useful on Windows, or for specifying a
                   non-default tesseract command. e.g. on Windows:
-                  C:/Program&nbsp;Files&nbsp;(x86)/Tesseract-OCR/tesseract.exe</p>
+                  C:/ProgramFiles(x86)/Tesseract-OCR/tesseract.exe</p>
                 </dd>
                 <dt><a name=
                 "RCL.INSTALL.CONFIG.RECOLLCONF.ABBYYLANG" id=