CMake: Update documentation of STRING(SUBSTRING) for length -1 (#10740)

Source/cmStringCommand.cxx

@@ -677,7 +677,7 @@ bool cmStringCommand::HandleSubstringCommand(std::vector<std::string> const&
   if ( end < -1 || end > leftOverLength )
     {
     cmOStringStream ostr;
-    ostr << "end index: " << end << " is out of range " << 0 << " - "
+    ostr << "end index: " << end << " is out of range -1 - "
          << leftOverLength;
     this->SetError(ostr.str().c_str());
     return false;

Source/cmStringCommand.h

@@ -110,7 +110,8 @@ public:
       "a file.\n"
       "TOUPPER/TOLOWER will convert string to upper/lower characters.\n"
       "LENGTH will return a given string's length.\n"
-      "SUBSTRING will return a substring of a given string.\n"
+      "SUBSTRING will return a substring of a given string. If length is "
+      "-1 the remainder of the string starting at begin will be returned.\n"
       "STRIP will return a substring of a given string with leading "
       "and trailing spaces removed.\n"
       "RANDOM will return a random string of given length consisting of "