Fix #10819 Document that executables etc can depend on library in same package

Also updates example in section 1.2.9.1 of the Cabal User Guide, accordingly.

Also fixes grammatical typo in section 1.2.9.1.

Author: mpilgrem

doc/cabal-package-description-file.rst

@@ -1353,6 +1353,15 @@ system-dependent values for these fields.
     from the same package can be specified with the shorthand syntax
     ``my-package:{my-library1,my-library2}``.
 
+    .. Note::
+
+       The :pkg-field:`build-depends` field of an executable section (from
+       version 1.8 of the Cabal specification), test suite section (from
+       version 1.10) or benchmark section (from version 1.14) can specify a
+       library in the same package by the name of the library (without a version
+       constraint). Cabal then treats the executable, test suite or benchmark as
+       if it were in another package that depended on the package.
+
     .. Note::
 
        Before version 3.4 of the Cabal specification, from version 2.0, a
@@ -2317,7 +2326,7 @@ Configurations
 
 Library and executable sections may include conditional blocks, which
 test for various system parameters and configuration flags. The flags
-mechanism is rather generic, but most of the time a flag represents
+mechanism is rather generic, but most of the time a flag represents a
 certain feature, that can be switched on or off by the package user.
 Here is an example package description file using configurations:
 
@@ -2378,8 +2387,9 @@ Example: A package containing a library and executable programs
 
     Executable test1
       Main-is:          T1.hs
-      Other-Modules:    Testing.Test1
-      Build-Depends:    base >= 4.2 && < 4.9
+      Build-Depends:
+        , base >= 4.2 && < 4.9
+        , Test1
       Default-Language: Haskell2010
 
       if flag(debug)