Gets or sets the fields this signature locks, written as the signature field's lock dictionary; null (the
default) writes none. A field that already has a lock dictionary is locked by it whatever this says, and cannot take another.
public PdfFieldLockOptions? FieldLock { get; set; }
ISO 32000-2 §12.7.5.5, Tables 235 and 236, and §12.8.2.4 (FieldMDP, Table 259). PDF 1.5; a lock's P, PDF 2.0.