Docs: correct and unify descriptions for as serialization formats

[novusnota]

It may be nice to have a single place for all as serialization options/formats. A /book/serialization, /book/cell-serialization, or just a proper section on the Cells, Builders, Slices page, i.e. somewhere on /book/cells.

In general, as should be viewed not as a part of the type, but rather as a guidance for the composition and parsing of Cells, which is used for .toCell() and .fromCell() functions in particular.

Additionally, cases like as remaining need further clarifications: https://docs.tact-lang.org/book/cells/#serialization-remaining. In particular, it isn't true that Builder or Slice types get stored in persistent state as is — they're stored and retrieved as Cells.

struct Yo { field1: Int as int7; field2: Builder as remaining }

// ...somewhere below, in a receiver/getter...:

let sliceX = beginCell()
    .storeInt(42, 7)
    .storeSlice(emptySlice())
    .storeSlice(emptySlice())
    .storeUint(20, 6)
    .asSlice();
let yo = Yo.fromSlice(sliceX);
yo.field1; // 42
yo.field2; // (two consecutive empty Slices and a 20) as a Builder