Clarify documentation for enable pseudo-variable

The documentation for variables says that for boolean variables the form
`name1+name2+name3:var` is equivalent to
`name1:var & name2:var & name3:var` but it's possible to write
`name1+name2:enable` despite `enable` being the string "enable" or
"disable". This change clarifies that `enable` isn't really a variable
but syntactic sugar instead.

doc/pages/Manual.md

@@ -492,9 +492,11 @@ the package being defined.
   resolved direct dependencies of the package
 - <a id="pkgvar-installed">`installed`</a>:
   whether the package is installed
-- <a id="pkgvar-enable">`enable`</a>:
-  takes the value "enable" or "disable" depending on whether the package is
-  installed
+- <a id="pkgvar-enable">`enable`</a>: is not a variable.
+  `pkg1+pkg2+...+pkgN:enable` is syntactic sugar for
+  `pkg1+pkg2+...+pkgN:installed?enable:disable`. That is, it behaves like a
+  variable that takes the value "enable" or "disable" depending on whether the
+  package(s) is installed
 - <a id="pkgvar-pinned">`pinned`</a>: whether the package is pinned
 - <a id="pkgvar-dirs">`bin`, `sbin`, `lib`, `man`, `doc`, `share`, `etc`</a>:
   the corresponding directories for this package (similar to

master_changes.md

@@ -113,6 +113,8 @@ users)
 
 ## Doc
 
+  * Clarify documentation for `enable` pseudo-variable
+
 ## Security fixes
 
 # API updates