Lua_api.txt: Various improvements

Improve documentation for formspec 'position' and 'anchor' elements.
Add missing documentation of 'animation' and 'glow' for particle spawners.

doc/lua_api.txt

@@ -1897,14 +1897,23 @@ examples.
* deprecated: `invsize[<W>,<H>;]`

#### `position[<X>,<Y>]`
* Define the position of the formspec
* A value between 0.0 and 1.0 represents a position inside the screen
* The default value is the center of the screen (0.5, 0.5)
* Must be used after `size` element.
* Defines the position on the game window of the formspec's `anchor` point.
* For X and Y, 0.0 and 1.0 represent opposite edges of the game window, for example:
    * [0.0, 0.0] sets the position to the top left corner of the game window.
    * [1.0, 1.0] sets the position to the bottom right of the game window.
* Defaults to the center of the game window [0.5, 0.5].

#### `anchor[<X>,<Y>]`
* Define the anchor of the formspec
* A value between 0.0 and 1.0 represents an anchor inside the formspec
* The default value is the center of the formspec (0.5, 0.5)
* Must be used after both `size` and `position` (if present) elements.
* Defines the location of the anchor point within the formspec.
* For X and Y, 0.0 and 1.0 represent opposite edges of the formspec, for example:
    * [0.0, 1.0] sets the anchor to the bottom left corner of the formspec.
    * [1.0, 0.0] sets the anchor to the top right of the formspec.
* Defaults to the center of the formspec [0.5, 0.5].

* `position` and `anchor` elements need suitable values to avoid a formspec
  extending off the game window due to particular game window sizes.

#### `container[<X>,<Y>]`
* Start of a container block, moves all physical elements in the container by (X, Y)
@@ -5277,6 +5286,10 @@ Note that in params, use of symbols is as follows:
    --  ^ Uses texture (string)
        playername = "singleplayer"
    --  ^ Playername is optional, if specified spawns particle only on the player's client
        animation = {Tile Animation definition},
    --  ^ optional, specifies how to animate the particle texture
        glow = 0
    --  ^ optional, specify particle self-luminescence in darkness
    }

### `HTTPRequest` definition (`HTTPApiTable.fetch_async`, `HTTPApiTable.fetch_async`)